The vertebrate adaptive immune systems (Agnatha and Gnathostomata) use sets of T and B lymphocyte lineages that somatically generate highly diverse repertoires of Ag-specific receptors and Abs. In Gnathostomata, cytokine networks regulate the activation of lymphoid and myeloid cells, whereas little is known about these components in Agnathans. Most gnathostome cytokines are four-helix bundle cytokines with poorly conserved primary sequences. In contrast, sequence conservation across bilaterians has been observed for cognate cytokine receptor chains, allowing their structural classification into two classes, and for downstream JAK/STAT signaling mediators. With conserved numbers among Gnathostomata, human cytokine receptor chains (comprising 34 class I and 12 class II) are able to interact with 28 class I helical cytokines (including most ILs) and 16 class II cytokines (including all IFNs), respectively. Hypothesizing that the arsenal of cytokine receptors and transducers may reflect homologous cytokine networks, we analyzed the lamprey genome and transcriptome to identify genes and transcripts for 23 class I and five class II cytokine receptors alongside one JAK signal mediator and four STAT transcription factors. On the basis of deduction of their respective orthologs, we predict that these receptors may interact with 16 class I and 3 class II helical cytokines (including IL-4, IL-6, IL-7, IL-12, IL-10, IFN-γ, and thymic stromal lymphoprotein homologs). On the basis of their respective activities in mammals, this analysis suggests the existence of lamprey cytokine networks that may regulate myeloid and lymphoid cell differentiation, including potential Th1/Th2 polarization. The predicted networks thus appear remarkably homologous to those of Gnathostomata, albeit reduced to essential functions.

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Adaptive immunity characterized by the somatic generation of Ag receptor repertoires has long been regarded as a special feature of Gnathostomata (jawed vertebrates), whereas innate immunity has been shown in all bilaterian animal lineages, with most studies being performed in arthropods and chordates. However, the discovery in the extant agnathan (jawless vertebrates, lampreys, and hagfishes) that variable lymphocyte receptors (VLRs) of the leucin-rich repeat (LRR) family are somatically generated by a process related to gene conversion revealed that specific adaptive immunity is in fact a hallmark of all vertebrates (110). The difference in Ag receptor types, Ig based versus LRR based, originally suggested a major divergence between the agnathan and gnathosotome immune systems. However, it was subsequently discovered that activation-induced cytidine deaminase–like cytidine deaminase enzymes could play similar roles in both vertebrate lineages. Moreover, three types of VLR—VLR-A, -B, and -C—are expressed by lymphocyte lineages homologous to the gnathostome lineages: VLRA- and VLRC-producing cells correspond to T cell α/β and γ/δ, respectively, whereas VLRB are produced by B cells and their plasma cell progeny (10). Furthermore, the existence of an organ with features of a primitive thymus was established (6). Consequently, the agnathan and gnathostome adaptive immune systems are much more closely related than originally thought. Accordingly, one could conjecture that coordination of the agnathan lymphocyte activation is controlled by similar and perhaps homologous cytokine networks.

In the Gnathostomata, the dichotomy between humoral (Th2) and cellular (Th1) adaptive immune responses is promoted by IL-4 and IL-12, respectively, to lead to subsequent production of IL-4 itself, IL-5, IL-10, or IL-13 for Th2 response and IL-2 or IFN-γ for Th1 response, all of those being helical cytokines (1115). The helical cytokines and their receptors are classified into two major classes according to their function or structure. Class I cytokines, which form the largest family, include a majority of ILs, IL-2, IL-3, IL-4, IL-5, IL-6, IL-7, IL-9, IL-11, IL-12, IL-13, IL-15, IL-23, IL-27, and IL-35, LIF, G-CSF, GM-CSF, thymic stromal lymphoprotein (TSLP), oncostatin M, growth hormone, prolactin (PRL), erythropoietin (EPO), and thrombopoietin (TPO). Receptor chains for class I cytokines have been further subclassified into five structural groups (16) (Fig. 1). Class II cytokines comprise IFNs of type I (IFN-α, IFN-β), type II (IFN-γ), and type III (IFN-λ, IL-28, and IL-29, and IL-10–related cytokines, i.e., IL-10, IL-19, IL-20, IL-22, IL-24, and IL-26). Structural relatedness between cognate cytokines was not revealed by primary amino acid sequence, although subsequent secondary and structural analyses indicated a common spatial organization in four-helix bundles (17, 18). This lack of sequence conservation, even within warm-blooded vertebrates, complicated the identification by sequence similarity in more distant species. Thus, strategies for cytokine ortholog identification were based mainly on synteny rather than on primary sequence comparison (19, 20). Fortunately, the resolution of downstream cytokine signaling cascades revealed that helical cytokine receptors harbor common structural features. Indeed, extracellular parts of cognate receptor chains commonly harbor two fibronectin-like domains, designated hematopoietin receptor (HpoR) domains, which are conserved among Bilateria (18, 2123). Receptor chains for helical class I cytokines, to which most ILs belong, have four conserved cysteines in β-sheets 1, 2, 4, and 5 of fibronectin-like motif 1 and a tryptophan-serine doublet at the edge of β-sheet 7 of fibronectin-like motif 2. Receptor chains for class II helical cytokines, which include all IFNs, show four conserved cysteines in β-sheets 4 and 5 of motif 1 and in β-sheets 6 and 7 of motif 2 (21) (Fig. 1). Ancestral helical cytokine receptor chains have been identified in arthropods and cnidarians (22, 24). Furthermore, highly conserved receptor-bound kinases designated JAKs and STAT factors were identified, with a full description of the JAK/STAT pathway in Drosophila involved, among others, in hematopoiesis, and the presence of STAT in Caenorhabditis elegans (22, 2427).

Across the different phyla, the number of elements may vary significantly from two receptors (with one JAK and one STAT in Drosophila), up to generally 35 class I and 12 class II receptor chains (four JAKs and seven STATs) in Gnathostomata (22, 26). This remarkable expansion may reflect the occurrence, specialization, and activation of effector cells involved in finely tuned adaptive immune responses and raises immediate questions about the phylogenetic origin of the diversity of cytokines, receptors, and signaling elements in Agnatha. There is currently limited information about cytokines in lamprey. Thus far, only members of the IL-17, TNF, and IL-1 families and chemokines have been identified via their conserved primary sequence motifs (2832). Because the main regulators of immunocyte diversity, T cell ontogeny, activation, and CD4 Th polarization are class I or class II helical cytokines, we have examined the sea lamprey (Petromyzon marinus) genome and transcriptome to predict the existence of such helical cytokines by identifying their cognate receptors. We found evidence for considerable expansion in the number of the cytokine receptors compared with nonchordates, but in substantially smaller numbers than in Gnathostomata. This state of receptor diversity in the sea lamprey offers insight into how fundamental cytokine networks may have arisen to modulate adaptive immune responses.

Lamprey protein hematopoietin receptor sequences were identified by BlastP search at the National Center for Biotechnology Information (NCBI) server (https://blast.ncbi.nlm.nih.gov/Blast.cgi) using human and shark sequences as queries on the lamprey RefSeq database produced from sperm mRNA. Genomic sequences were isolated by TBlastN search on the NCBI server from the lamprey P. marinus whole-genome shotgun sequencing project database (accession PIZI01000000 and relevant contigs publicly available at https://www.ncbi.nlm.nih.gov/nuccore/PIZI00000000.1/), the second-generation P. marinus genomic scaffolds (PIZI series) constructed from germline DNA (33). P. marinus chromosome assignment was done by TBlastN on the Genome (kPetMar1.pri reference Annotation release 100) database on the NCBI Genome server. Additional RefSeq protein and genome screenings involved sharks and rays with a primary focus on the elephant shark Callorhinchus milii, including the phylogenetically basal bony fish Lepisosteus oculatus, coelacanth Latimeria chalumnae, and tunicate Ciona intestinalis RefSeq (3437). Gene locations were extracted from the Genomicus server (https://www.genomicus.biologie.ens.fr/) and using TBlastN searches.

Predicted peptide sequences were subject to multiple sequence alignment using PSI/TM-Coffee with UniRef100 homology extension at the Swiss Institute for Bioinformatics server https://tcoffee.crg.eu/apps/tcoffee/index.html. The sequence alignments were further adjusted manually if necessary. Phylogenic trees were constructed by the neighbor-joining method (38) using the Simple Phylogeny program on the European Molecular Biology Laboratory–European Bioinformatics Institute server (https://www.ebi.ac.uk/Tools/phylogeny/simple_phylogeny/).

Fibronectin-like extracellular domains of class I helical cytokine receptors (i.e., IgC2, hematopoietin receptor domains 1 and 2, and fibronectin type III) were individually analyzed for secondary structures using SwissModel (https://swissmodel.expasy.org). Multiple sequence alignments were further adjusted manually if necessary. Highlights are shown in Supplemental Fig. 2.

Screening of P. marinus protein RefSeq (see above) and whole-genome shotgun sequence databases using class I cytokine receptor query sequences revealed 21 individual cDNA entities and two additional genomic candidates for which no cDNA counterpart was found (Table I). We also found that the P. marinus genome encodes five class II receptor chains, considering three cDNAs showing extremely high sequence conservation as a single entity (Supplemental Fig. 2). Corresponding genes are widely spread in the lamprey genome, with limited gene clustering (Table I, Fig. 2, Supplemental Fig. 3).

FIGURE 1.

Structure-based classification of Class I (left) and Class II (right) hematopoietin receptor chains. Based on (16), color code for the six receptor chain groups is consistent throughout the report. Individual fibronectin III–derived domains (IgC2, hematopoietin, and fibronectin III) (21) are shown by smoothened squares with conserved amino acids as horizontal bars. Positions of intracellular tyrosines are random.

FIGURE 1.

Structure-based classification of Class I (left) and Class II (right) hematopoietin receptor chains. Based on (16), color code for the six receptor chain groups is consistent throughout the report. Individual fibronectin III–derived domains (IgC2, hematopoietin, and fibronectin III) (21) are shown by smoothened squares with conserved amino acids as horizontal bars. Positions of intracellular tyrosines are random.

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FIGURE 2.

Expansion of Class I (left), Class II helical cytokine receptors, and receptor-associated SUSHI and JAK/STAT signaling (right) genes in chordate evolution. Gene clustering is indicated by shaded areas. In humans, physical distances in kb or Mb and chromosome band location are shown. Because possible gaps in the C. milii genome and transcriptome may generate unfound genes and cDNAs, missing C. milii orthologs were “compensated” by chondrichthyan sequences found in Rhincodon typus (open circle), Amblyraja radiata (open triangle), or closely related bony fish Lepisosteus oculeatus (open square). C. i., Ciona intestinalis; P. m., Petromyzon marinus; C. m., Callorhinchus milii; L. c., Latimeria chalumnae.

FIGURE 2.

Expansion of Class I (left), Class II helical cytokine receptors, and receptor-associated SUSHI and JAK/STAT signaling (right) genes in chordate evolution. Gene clustering is indicated by shaded areas. In humans, physical distances in kb or Mb and chromosome band location are shown. Because possible gaps in the C. milii genome and transcriptome may generate unfound genes and cDNAs, missing C. milii orthologs were “compensated” by chondrichthyan sequences found in Rhincodon typus (open circle), Amblyraja radiata (open triangle), or closely related bony fish Lepisosteus oculeatus (open square). C. i., Ciona intestinalis; P. m., Petromyzon marinus; C. m., Callorhinchus milii; L. c., Latimeria chalumnae.

Close modal
Table I.

Types I and II helical receptors, downstream signaling JAK/STAT mediators, neighboring genes encoded by lamprey P. marinus genome, and presumed orthologs in human and C. intestinalis

Lamprey CandidatesGene PositionsPredicted DomainsPresumed Human OrthologPresumed Ciona Ortholog
Receptor ChainsGene Product RefSeqLength aaSyntenic LinkgDNA Contig wgsPosition kbChrAccession No.Position kbIgC2 aaHpoR1 aaHpoR2 aaFnIII aaTM aaJak Docking aaCP aaAccession No. RefSeqLength aa% Identity BlastP% Positivity BlastPAccession RefSeqLength aaChrAccession No.Position
Group 5 *GenBank                          
 XP_032827897 453  PIZI01000001 13969–979 47 NC_046115 644–654 26–140 141–246 247–343 no 344–382 383–453 no IL13RA NP_001551 427 26 44       
   HTR2C PIZI01000001 13411–374                      
 XP_032836216 409  PIZI01000429 63–60 64 NW_022638124 195–177 no 41–162 163–260 no 261–295 296–341 342–409 IL2RG NP_000197 369 N/A N/A       
 XP_032827687 657  PIZI01000001 19516–501 47 NC_046115 7156–7140 20–106+ 218–396 397–503 no 504–561 562–606 607–657 CRLF2 NP_071431 371 35 51       
         107–217                  
Group 4   SHOX PIZI01000001 18860–897                      
 XP_032811611 924  PIZI01000025 13135–145 17 NC_046085 759–749 no 78–177+ 178–277+ no 473–502 503–569 570–924 IL4RA NP_000409 825 N/A N/A       
          278–368 369–472                
Group 1 not found   PIZI01000043 4518 44 NC_046112 4814–4815  yes yes   yes  IL7RA NP_002176 459 N/A N/A       
   SPEF2 PIZI01000043 4541–561                      
 XP_032826436 736  PIZI01000043 4707–655 44 NC_046112 5′021–4974 no 64–183 184–294 no 295–324 325–397 398–736 PRLR NP_000940 622 29 44       
 AXB87817* 651  PIZI01000023 11739–622 35 NC_046103 12122–12116 no 48–149 150–259 no 260–290 291–366 367–651 GHR NP_000154 638 33 53       
 XP_032822207 840  PIZI01000023 12321–110 35 NC_046103 12540–12589  46–177+ 178–290+ no 501–527 528–600 601–840 TPOR NP_005364 635 25 39       
          291–391 392–500                
 XP_032823247 436  PIZI01000037 10094–102 37 NC_046105 10443–10453  61–133 134–278 no 279–314 315–392 393–436 CRLF3 NP_057070 442 46 62 CRLF3 NP_001107600 463 Unknown NW_004190439  
Group 2   ATAD5 PIZI01000037 10088–059                      
 XP_032822078 988  PIZI01000023 6864–834 35 NC_046103 7099–7069 28–135 136–250 251–355 356–653 654–689 690–776 777–988 LPTR NP_002294 1165 25 41 LPTR NP_001107599 1123 NC_020170 3′769–780k 
 XP_032821122 712  PIZI01000029 1814–787 33 NC_046101 11617–11599 20–169 170–268 269–369 370–676 677–712 704–712  IL12RB2 NP_001550 862 25 40       
 XP_032821795 982  PIZI01000023 1087–131 35 NC_046103 1172–1296 33–112 113–164 165–262 263–614 615–636 637–721 722–982 IL12RB1 NP_005526 662 N/A N/A       
 XP_032812536 1021  PIZI01000006 8912–893 NC_046069 22210–22192 23–133 134–242 243–360 361–714 715–736 737–825 826–1021 IL6RB NP_002175 975 23 38       
Group 3 XP_032830211 831  PIZI01000095 758–732 53 NC_046121 123–144 21–110 111–205 206–317 318–641 642–672 673–751 752–831 LIFR NP_002301 1097 24 42       
 XP_032824781 425  PIZI01000031 10317–303 40 NC_046108 2113–2124 1–102   211–258 No 259–425 IL12BC-L1 NP_002178 328 30 44       
 XP_032824670 516  PIZI01000031 10292–269 40 NC_046108 2134–2162 39–145 146–240 241–341  342–390 No 391–516 IL12BC-L2 NP_002178 328 32 48       
 XP_032809111 487  PIZI01000013 12819–803 13 NC_046081 5′520–5′536 34–117 118–210 211–318  319–370 No 371–487 IL12BC-L3 NP_002178 328 N/A N/A       
 XP_032821830 372  PIZI01000023 1764–749 35 NC_046103 1822–1806 31–123 124–221 222–313  324–346 No 347–372 CRLF-1 NP_004741 422 40 53       
 XP_032809575 458  PIZI01000102 1011–023 13 NC_046081 15285–15273 37–126 127–237 229–340  341–404 No 405–458 IL27BC-L1 NP_005746 229 31 48       
 XP_032829729 422  PIZI01000052 5′278–307 52 NC_046120 3′565–3′594 25–129 130–228 229–332  333–380 No 381–422 IL27BC-L2 NP_005746 229 29 48       
 not found   PIZI01000006 16942–947 NC_046069 30362–30367  Yes Yes     IL6RA NP_000556 468 N/A N/A       
 XP_032806009 618  PIZI01000006 16975–990 NC_046069 30397–30410 21–109+ 110–204+ 205–315+  No No No IL11RA NP_001136256 422 29 43       
         316–403 404–503 504–618                
 XP_032806040 594  PIZI01000006 17001–010 NC_046069 30421–30430 18–108+ 110–198+ 199–298+  569–594 No No CNTFRA NP_001833 372 34 52       
         299–316 377–473 474–568                
Class II   GALT PIZI01000006 17031–026                      
 XP_032826345 527  PIZI01000011 8362–368 33 NC_046011 6569–6576  38–137 138–237  238–267 268–335 336–871 IFNGR1 NP_000407 489 N/A N/A IFNGR1 XP_026692289 871 10 NC_020175 2244–234k 
 XP_032828432 361  PIZI01000001 25709–701 49 NC_046117 3′065–3′065  58–158 159–261  262–297 298–347 348–361 IFNAR1 NP_001371427 558 N/A N/A IFNAR1 XP_002128492 505 10 NC_020175 2229–234k 
 XP_032815494 348  PIZI01000006 918–925 NC_046069 14107–14115  24–141 142–255  256–282 283–332 333–348 IFNLR1 NP_734464 520 N/A N/A       
 XP_032800330 486    Unknown NW_022639082 82–89  29–133 134–348  349–369 370–430 431–486 IL10RA NP_001549 578 N/A N/A       
 XP_032801345 467    Unknown NW_022639082 80–72  33–137 138–349  350–377 378–446 447–467 IL10RA           
 XP_032800329 466    Unknown NW_022639082 11.Nov  29–133 134–348  349–376 377–445 446–466 IL10RA           
 XP_032812875 295  PIZI01000006 9144–9156 NC_046069 22525–22536  28–129 130–236  237–295   F3/TF NP_001984 295 33 50       
 XP_032801826  ABCD3 PIZI01000006 7774–7750 NC_046069 21015–20990                   
JAKs                           
paleo-JAKs                           
 XP_032826686   PIZI01000043 3′150–530k 44 NC_046112 3′460–530        JAK1 NP_002218 1154 41 57       
neo-JAKs   AK4 PIZI01000043 3′104–095k                      
 not found                          
STATs                           
paleo-STATs                           
 XP_032813199 792  PIZI01000028 11193–176k 19 NC_046087 12297–280        STAT5A NP_003143 794 67 81       
                STAT5B NP_036580 787 68 81       
 XP_032815009 620  PIZI01000005 3′746–716k NC_046070 4374–343        STAT6 NP_003144 847 46 62       
neo-STATs   GLS PIZI01000005 3′791–833k                      
 XP_032814978 771  PIZI01000005 3′869–850k NC_046070 4500–478        STAT1 NP_009330 750 55 70       
 XP_032802147 828  PIZI01000004 10694–680k NC_046072 15263–279        STAT4 NP_003142 748 53 70       
   MYO1B PIZI01000004 10741–792k                      
Lamprey CandidatesGene PositionsPredicted DomainsPresumed Human OrthologPresumed Ciona Ortholog
Receptor ChainsGene Product RefSeqLength aaSyntenic LinkgDNA Contig wgsPosition kbChrAccession No.Position kbIgC2 aaHpoR1 aaHpoR2 aaFnIII aaTM aaJak Docking aaCP aaAccession No. RefSeqLength aa% Identity BlastP% Positivity BlastPAccession RefSeqLength aaChrAccession No.Position
Group 5 *GenBank                          
 XP_032827897 453  PIZI01000001 13969–979 47 NC_046115 644–654 26–140 141–246 247–343 no 344–382 383–453 no IL13RA NP_001551 427 26 44       
   HTR2C PIZI01000001 13411–374                      
 XP_032836216 409  PIZI01000429 63–60 64 NW_022638124 195–177 no 41–162 163–260 no 261–295 296–341 342–409 IL2RG NP_000197 369 N/A N/A       
 XP_032827687 657  PIZI01000001 19516–501 47 NC_046115 7156–7140 20–106+ 218–396 397–503 no 504–561 562–606 607–657 CRLF2 NP_071431 371 35 51       
         107–217                  
Group 4   SHOX PIZI01000001 18860–897                      
 XP_032811611 924  PIZI01000025 13135–145 17 NC_046085 759–749 no 78–177+ 178–277+ no 473–502 503–569 570–924 IL4RA NP_000409 825 N/A N/A       
          278–368 369–472                
Group 1 not found   PIZI01000043 4518 44 NC_046112 4814–4815  yes yes   yes  IL7RA NP_002176 459 N/A N/A       
   SPEF2 PIZI01000043 4541–561                      
 XP_032826436 736  PIZI01000043 4707–655 44 NC_046112 5′021–4974 no 64–183 184–294 no 295–324 325–397 398–736 PRLR NP_000940 622 29 44       
 AXB87817* 651  PIZI01000023 11739–622 35 NC_046103 12122–12116 no 48–149 150–259 no 260–290 291–366 367–651 GHR NP_000154 638 33 53       
 XP_032822207 840  PIZI01000023 12321–110 35 NC_046103 12540–12589  46–177+ 178–290+ no 501–527 528–600 601–840 TPOR NP_005364 635 25 39       
          291–391 392–500                
 XP_032823247 436  PIZI01000037 10094–102 37 NC_046105 10443–10453  61–133 134–278 no 279–314 315–392 393–436 CRLF3 NP_057070 442 46 62 CRLF3 NP_001107600 463 Unknown NW_004190439  
Group 2   ATAD5 PIZI01000037 10088–059                      
 XP_032822078 988  PIZI01000023 6864–834 35 NC_046103 7099–7069 28–135 136–250 251–355 356–653 654–689 690–776 777–988 LPTR NP_002294 1165 25 41 LPTR NP_001107599 1123 NC_020170 3′769–780k 
 XP_032821122 712  PIZI01000029 1814–787 33 NC_046101 11617–11599 20–169 170–268 269–369 370–676 677–712 704–712  IL12RB2 NP_001550 862 25 40       
 XP_032821795 982  PIZI01000023 1087–131 35 NC_046103 1172–1296 33–112 113–164 165–262 263–614 615–636 637–721 722–982 IL12RB1 NP_005526 662 N/A N/A       
 XP_032812536 1021  PIZI01000006 8912–893 NC_046069 22210–22192 23–133 134–242 243–360 361–714 715–736 737–825 826–1021 IL6RB NP_002175 975 23 38       
Group 3 XP_032830211 831  PIZI01000095 758–732 53 NC_046121 123–144 21–110 111–205 206–317 318–641 642–672 673–751 752–831 LIFR NP_002301 1097 24 42       
 XP_032824781 425  PIZI01000031 10317–303 40 NC_046108 2113–2124 1–102   211–258 No 259–425 IL12BC-L1 NP_002178 328 30 44       
 XP_032824670 516  PIZI01000031 10292–269 40 NC_046108 2134–2162 39–145 146–240 241–341  342–390 No 391–516 IL12BC-L2 NP_002178 328 32 48       
 XP_032809111 487  PIZI01000013 12819–803 13 NC_046081 5′520–5′536 34–117 118–210 211–318  319–370 No 371–487 IL12BC-L3 NP_002178 328 N/A N/A       
 XP_032821830 372  PIZI01000023 1764–749 35 NC_046103 1822–1806 31–123 124–221 222–313  324–346 No 347–372 CRLF-1 NP_004741 422 40 53       
 XP_032809575 458  PIZI01000102 1011–023 13 NC_046081 15285–15273 37–126 127–237 229–340  341–404 No 405–458 IL27BC-L1 NP_005746 229 31 48       
 XP_032829729 422  PIZI01000052 5′278–307 52 NC_046120 3′565–3′594 25–129 130–228 229–332  333–380 No 381–422 IL27BC-L2 NP_005746 229 29 48       
 not found   PIZI01000006 16942–947 NC_046069 30362–30367  Yes Yes     IL6RA NP_000556 468 N/A N/A       
 XP_032806009 618  PIZI01000006 16975–990 NC_046069 30397–30410 21–109+ 110–204+ 205–315+  No No No IL11RA NP_001136256 422 29 43       
         316–403 404–503 504–618                
 XP_032806040 594  PIZI01000006 17001–010 NC_046069 30421–30430 18–108+ 110–198+ 199–298+  569–594 No No CNTFRA NP_001833 372 34 52       
         299–316 377–473 474–568                
Class II   GALT PIZI01000006 17031–026                      
 XP_032826345 527  PIZI01000011 8362–368 33 NC_046011 6569–6576  38–137 138–237  238–267 268–335 336–871 IFNGR1 NP_000407 489 N/A N/A IFNGR1 XP_026692289 871 10 NC_020175 2244–234k 
 XP_032828432 361  PIZI01000001 25709–701 49 NC_046117 3′065–3′065  58–158 159–261  262–297 298–347 348–361 IFNAR1 NP_001371427 558 N/A N/A IFNAR1 XP_002128492 505 10 NC_020175 2229–234k 
 XP_032815494 348  PIZI01000006 918–925 NC_046069 14107–14115  24–141 142–255  256–282 283–332 333–348 IFNLR1 NP_734464 520 N/A N/A       
 XP_032800330 486    Unknown NW_022639082 82–89  29–133 134–348  349–369 370–430 431–486 IL10RA NP_001549 578 N/A N/A       
 XP_032801345 467    Unknown NW_022639082 80–72  33–137 138–349  350–377 378–446 447–467 IL10RA           
 XP_032800329 466    Unknown NW_022639082 11.Nov  29–133 134–348  349–376 377–445 446–466 IL10RA           
 XP_032812875 295  PIZI01000006 9144–9156 NC_046069 22525–22536  28–129 130–236  237–295   F3/TF NP_001984 295 33 50       
 XP_032801826  ABCD3 PIZI01000006 7774–7750 NC_046069 21015–20990                   
JAKs                           
paleo-JAKs                           
 XP_032826686   PIZI01000043 3′150–530k 44 NC_046112 3′460–530        JAK1 NP_002218 1154 41 57       
neo-JAKs   AK4 PIZI01000043 3′104–095k                      
 not found                          
STATs                           
paleo-STATs                           
 XP_032813199 792  PIZI01000028 11193–176k 19 NC_046087 12297–280        STAT5A NP_003143 794 67 81       
                STAT5B NP_036580 787 68 81       
 XP_032815009 620  PIZI01000005 3′746–716k NC_046070 4374–343        STAT6 NP_003144 847 46 62       
neo-STATs   GLS PIZI01000005 3′791–833k                      
 XP_032814978 771  PIZI01000005 3′869–850k NC_046070 4500–478        STAT1 NP_009330 750 55 70       
 XP_032802147 828  PIZI01000004 10694–680k NC_046072 15263–279        STAT4 NP_003142 748 53 70       
   MYO1B PIZI01000004 10741–792k                      

We classified class I receptor cDNAs into the five structural groups defined earlier (16). Groups 1 and 4 receptors solely carry HpoR domains 1 and 2 as extracellular domains, which are duplicated in a few cases (Fig. 1). Groups 2 and 3 receptors carry an additional IgC2 motif at the N-terminus, with two conserved cysteines in β-sheets 2 and 6 and one tryptophan in β-sheet 3. The N-terminal motif of group 5 receptors appears more divergent from IgC2, with the first cysteine replaced by a tryptophan, the second cysteine replaced by a valine, and absence of a conserved tryptophan (Table I, Fig. 1, Supplemental Fig. 2). The presence of fibronectin type III domains, usually three, between HpoR and transmembrane domains, and a long cytoplasmic domain is a hallmark of group 2 (Table I, upper right; and (Fig. 1). Further classification was based on the existence of a transmembrane region and the length of intracellular domains. Juxtamembrane JAK docking site motifs (39, 40) are present in all receptor chains, except those in group 3. Indeed, group 3 chains show short cytoplasmic domains devoid of JAK-anchoring sites or no transmembrane domain at all. To the former belong receptor α-chains, such as IL-6, whereas the latter consist of the β-chain subunits for helical IL-12 or IL-27 α-chains to construct IL-12, IL27, or IL-35 holocytokines. Class II receptor chains were treated as a single homogeneous group.

Orthologs of only some of the individual receptor chain genes could be identified by synteny. The conserved vicinity of the HTR2C gene with IL13RA2 in mammals allowed the definition of the receptor chain located nearby lamprey HTR2C as IL13RA related (Table I, upper left). Similarly, we defined CRLF2 (cytokine receptor-like factor 2), IL7RA, GHR (growth hormone receptor), CRLF3, IL6RA, IL11RA, and CNTFRA (ciliary neurotrophic factor receptor α-chain), and F3/TF (tissue factor) orthologs (Table I, upper left). Within each group, phylogenetic analysis of lamprey and gnathostome chains only designate a few lamprey orthologs (e.g., CRLF3 in group 1, leptin receptor [LPTR] in group 2, or CRLF1 in group 3) (Supplemental Fig. 1). In fact, lamprey chains sometimes show higher similarity between themselves (possibly because of recent duplication) and more divergence relative to other gnathostome sequences (as, e.g., for GP130 and IL12RB1 in group 2; IL13RA, IL2RG, and CRLF2 in group 5; or IFNLR1 and IL10RA in class II) (Supplemental Fig. 1). Finally, local conservation of intracytoplasmic potential phosphorylation sites (tyrosine residues and neighboring amino acids) turned out to be critical to propose complete receptor chain ortholog classification, as detailed below (Table I, Supplemental Fig. 2).

Group 1 receptors

The number of lamprey group 1 chains (depicted in violet in (Figs. 13) is nearly constant between lamprey and gnathostomes, with the notable absence of the EPO receptor (EPOR) as an exception. This result indicates that the gene duplication in the TPOR (thrombopoietin receptor)/EPOR lineage may have occurred later in evolution. In contrast, CRLF3 shows a high degree of sequence conservation (Supplemental Fig. 2). CRLF3 identification in a cnidarian is consistent with the hypothesis that CRLF3 represents an ancestral form of these receptor chains in animal evolution (24). Our phylogenetic analysis also suggests that one cannot claim definitive orthology between lamprey GHR and PRL receptor (PRLR) genes (41) (Supplemental Fig. 2).

FIGURE 3.

Predicted spectrum of helical cytokines existing in lamprey. Based on human/mouse ligand/receptor binding combinations compiled in (26, 45). Receptor chains found from transcript and genomic P. marinus screening are depicted by smoothened squares (with extracellular N-terminal parts containing the fibronectin-like motifs oriented toward center) and predicted helical cytokines depicted by circles. Shaded areas indicate gene clustering in P. marinus genome and hypothesized earlier local duplication. Bold boxes indicate receptor chains subject to hypothesized later tandem duplication. Dotted outlines indicate genomic segments for which no RefSeq cDNA has been found. Boxed gene names indicate existence of orthologs in C. intestinalis.

FIGURE 3.

Predicted spectrum of helical cytokines existing in lamprey. Based on human/mouse ligand/receptor binding combinations compiled in (26, 45). Receptor chains found from transcript and genomic P. marinus screening are depicted by smoothened squares (with extracellular N-terminal parts containing the fibronectin-like motifs oriented toward center) and predicted helical cytokines depicted by circles. Shaded areas indicate gene clustering in P. marinus genome and hypothesized earlier local duplication. Bold boxes indicate receptor chains subject to hypothesized later tandem duplication. Dotted outlines indicate genomic segments for which no RefSeq cDNA has been found. Boxed gene names indicate existence of orthologs in C. intestinalis.

Close modal

Group 2 receptors

In humans, eight group 2 members can be paired on the basis of strong structural relatedness and gene colocation: IL12RB1/IL27R (19p13); IL12RB2/IL23R (1p31); IL-6 signal transducer (IL6ST or GP130)/IL31R (5q11), and leukemia-inhibiting factor receptor (LIFR)/oncostatin M receptor (5p13). We identified five lamprey group 2 members (red in (Figs. 13). Local sequence alignments of the cytoplasmic domain, which include JAK docking sides and conserved cytoplasmic residues that are potential tyrosine phosphorylation targets, proved to be a determinant to discriminate receptor chains from one another. This allowed classification of lamprey orthologs for IL12RB1, IL12RB2, IL6ST, and LIFR (those subject to further local tandem gene duplication) and the remaining one as lamprey LPTR (Supplemental Figs. 2 and 3). Being the closest relative of Drosophila DOME, LPTR may represent the ancestral group 2 receptor chain (Supplemental Figs. 2 and 3) (22).

Group 3 receptors

Surprisingly, the number of group 3 receptor chains (yellow in (Figs. 13) is higher in the lamprey than in any gnathostomian genome (9 versus 6). Among them are IL6RA (with genomic evidence only), IL11RA, CNTFR, and CRLF1, with clustering of IL6RA, IL11RA, and CNTFRA, and conserved location of CRLF1 next to ILR12RB1 between lamprey and human. Interestingly, we found three IL12BC-like and two IL27BC-like potential orthologs that showed unexpected transmembrane and short cytoplasmic tails not present in mammalian counterparts (Supplemental Fig. 2). Thus, they do not represent potential β-chains for IL-12, IL-27, or IL-35 helical components to generate active holocytokines according to the mammalian models. Rather, they are more likely to be α-chain receptors that bind helical subunits of IL-12 and IL-27 regarded as active cytokines, because the IL-6RA α-chain would bind IL-6.

Group 4 receptors

Regarding group 4 (blue in Figs. 13), we found a genomic IL7RA ortholog but no cDNA counterpart. In addition, we identified a candidate cDNA encoding a long cytoplasmic tail with tyrosines that could be aligned with those of human IL4RA interacting with IRS2 (Y497) STAT6 (Y575, Y603, Y631) and an ITIM motif (Y713) (42) (Supplemental Fig. 2). Conservation of these critical residues for IL-4 signaling suggests that the receptor encoded by this cDNA is likely to be derived from the presumed ancestor of IL4RA. Like human IL3RB, it carries duplication of both HpoR domains, suggesting that IL3RB derives from duplication of this ancestral IL4RA. In shark and coelacanth genomes, clustering of α-chain receptor genes IL4RA, IL21RA, and IL9RA on the one hand and β-chain receptor genes IL3RB and IL2RB on the other hand suggests two rounds of duplications, one being local tandem and one being more ancient (Fig. 2). IL9RA appears isolated from the IL4RA/IL21RA 16p11 cluster in the human genome as if the IL9RA gene region has been split (Fig. 2, left). Because we found no evidence for IL2RB, IL2RA, and IL15RA in sea lamprey, lampreys may lack IL-2/IL-15 cytokines.

Group 5 receptors

We found three lamprey group 5 receptors chains (green in Figs. 13). Besides orthologs of IL13RA and a CRLF2/CSF2RA/IL3RA/IL5RA homolog assigned by synteny, we defined IL2RG by sequence homology with Gnathostomata counterparts (Supplemental Fig. 2). As in the above description, in the shark genome, CRLF2 is located close to SHOX as part of a gene complex that also contains CSF2RA, IL3RA, and IL5RA, whereas IL5RA is separated from the other genes in coelacanth and human genomes (Fig. 2, left). Nevertheless, on the basis of a cytoplasmic tyrosine-containing box at the CRLF2 C-terminus that is absent in CSF2RA, IL3RA, and IL5RA (Supplemental Fig. 2), the lamprey chain most probably represents the CRLF2 ortholog and is likely to be derived from the common ancestor of CSF2RA, IL3RA, and IL5RA (Fig. 2, left).

Class II receptors

Among the cDNAs for five lamprey class II receptor chains, we found (gray in Figs. 13) an F3/TF ortholog that is unambiguously defined, given its gene location close to ABCD3 and high interspecific sequence conservation (Table I, Supplemental Figs. 1 and 2). In humans, nine IFN receptor genes map to three clusters, 21q22 (IFNAR1, IFNAR2, IFNGR2, IL10RB), 6q23 (IFNGR1, IL20RA, IL22RA1), and 1p36 (IFNLR1, IL22RA1), and two further genes are isolated on 11q23 (IL10RA) and 3q22 (IL20RB) (Table I, Fig. 2, and Supplemental Fig. 3). Similar to what we observed for most class I receptor chains, three predicted lamprey receptor sequences showed best alignments with single representatives of each human gene cluster, IFNAR1, IFNGR1, and IFNLR1, suggesting that 21q22, 6q23, and 1p36 clusters result from later tandem duplications. Finally, one showed the best alignment with IL10RA (Supplemental Fig. 2). Again, because of weak sequence similarity in the cytoplasmic domains, phylogenetic analysis did not reveal clear orthologs, whereas local conservation in cytoplasmic domains in the vicinity of potentially phosphorylation sites has helped in the attempt to solve this issue (Supplemental Figs. 1 and 2).

Regarding downstream cytokine signal transducers, a unique JAK representative has been found in lamprey, whereas the complete set of the JAKs exists in C. milii (Fig. 2, right). The lamprey JAK gene maps close to AK4 and therefore represents a JAK1 ortholog (Table I). Structurally, JAK1 and TYK2 show closer relatedness to ancestral JAK, such as Drosophila HOP, rather than to JAK2 or JAK3 (Supplemental Fig. 1). We therefore designated the former “paleo-JAKs” and the latter “neo-JAKs” (Fig. 2, right). We also found four sea lamprey STAT cDNAs that represent orthologs for STAT5, STAT6, also present in Ciona, as well as STAT1 and STAT4. As in humans, we found STAT4 mapping close to MYOB and STAT1 close to GLS, but on different chromosomes (2 and 4, respectively) (Table I). STAT5/STAT6 on the one hand and STAT1/STAT4 on the other hand are structurally related; therefore, we designated them “paleo-STATs” and “neo-STATS,” respectively (Fig. 2, right), whereas STAT3 and STAT2 are more related to STAT4 (Supplemental Fig. 1). Although JAK is unique in lamprey and expands to four paralogs in Gnathostomata, STAT expansion is progressive between one paralog in Drosophila, two in Ciona, four in lamprey, five in shark, six in coelacanth, seven in humans, and variable numbers among teleost species (43).

Sharing single chains to form specific receptor combinations for cognate ligands is a strategy that confers pleiotropic and redundant functions of cytokines (44). Based on the knowledge from human and mouse interactions (26, 45), the identification of individual receptor chains provides a means to predict the existence of cognate cytokines.

Group 1 cognate ligands

Group 1 receptor chains homodimerize upon ligand binding. This predicts the existence in lamprey of growth hormone, PRL, and TPO, whereas no ligand has been identified so far for orphan CRFL3 (Fig. 3).

Group 2/3 cognate ligands

Although LPTR homodimerizes upon leptin binding, heterodimerization of IL6ST/GP130 and LIFR, IL12RB1, and IL12RB2 chains also suggests the existence of LIF and IL-12/-27/-35 helical α-chains. Furthermore, considering association of group 3 as α-chains and group 2 as β-chains, the existence of IL-6, IL-11, and CNTF is expected (Fig. 3). The three putative IL12BC chains and two putative IL27BC chains are predicted to bind primarily to IL-12 and IL-27 α-helical components to form IL-12/-27/-35 holocytokines. However, on the basis of a prediction of transmembrane domains, they would rather represent IL-12 and IL-27 α-chain receptors, similar to closely related IL-6, IL-11, or CNTF, than cytokine β-chain subunits to form holocytokines. This suggests that helical subunits may represent ancestrally active forms of IL-12 and IL-27 that bind to their cognate HpoR-related β-chains.

Group 4/5 cognate ligands

With the combinations IL4RA/IL2RG and IL4RA/IL13RA, one could predict the existence of IL-4 and, possibly, closely related IL-13, genes that are likely to result from local tandem IL4-like duplication. Considering this chain as a common ancestor also for IL3RB, its combination with CRLF2 might bind an ancestral CSF2/IL-3/IL-5–related cytokine, whereas IL7RA/IL2RG and IL7RA/CRLF2 combinations may bind IL-7 and TSLP, respectively (Fig. 3).

Type II cognate ligands

Presumed orthologs for four lamprey class II cytokine receptors are defined as α-chains in humans; in other words, with no apparent evidence of β-chains. Interestingly, the human 21q22 cluster also includes IFNRA2, IFNRGR2, and IL10RB, all described as β-chain receptors and likely to result from IFNRA1 tandem duplication. If so, IFNRA1 might be regarded as an ancestral β-chain receptor commonly shared with IFNRG, IL10RA, and IFNLR1 α-chains. On the basis of mammalian IFN binding modalities, if a combination IL10RA/IFNAR1 receptor binds IL-10, IFNGR1/IFNAR1 and IFNLR1/IFNAR1 may bind IFN-γ and IFN-λ, respectively. However, IFNGR1/IFNAR1 and IFNLR1/IFNAR1 combinations may also bind multiple ligands, such as IFNs (IFN-α, -β, -δ, -ε, -κ, -ω), and IL-28 or IL-29, respectively (Fig. 3).

Our analysis indicates that the sea lamprey genome encodes at least 23 chains for class I and five chains for class II helical cytokine receptors. The corresponding genes are distributed across the lamprey genome in clusters on four chromosomes (22 receptor genes) and as isolated loci (6 receptor genes) (Table I). As a gold standard for orthology assignation, syntenic gene location could be established for only nine chains (i.e., IL13RA, CRLF2, IL7RA, GHR, CRLF3, IL6RA, IL11RA, and CNTFRA) and F3/TF. By default, the orthology of the remaining receptor chains had to rely on primary sequence similarities, the presence of additional extracellular elements such as IgC2 or fibronectin-like structures, and the length of cytoplasmic domains, as defined by the receptor chain classification we proposed earlier (16) (Fig. 1). Phylogenetic classification has also been used herein for classifying CRLF1, LPTR1, and, to some extent, PRLR and LIFR. Conservation of potentially phosphorylated cytoplasmic tyrosines and surrounding amino acids throughout evolution, although difficult to quantify, may also be a useful marker to differentiate individual receptor chains from one another. The functions of these proposed orthologs will require future experimental validation.

The receptor repertoire that we identify in the sea lamprey has arisen within a complex genomic history. The number of receptors in sea lamprey are intermediate between those identified in C. intestinalis (two receptors for class I and two for class II) and numbers that are typical for gnathostomes (e.g., 34 class I and 12 class II in humans). The increased multiplicity of receptors in vertebrates relative to the protochordates emerges from a combination of whole-genome duplication events, tandem gene duplications, and possibly gene loss in the ascidian lineage. One interpretation is that with an intermediate number of receptor chains, the sea lamprey genome provides an evolutionary snapshot of this receptor family expansion in early vertebrates. However, present-day lampreys have evolved independently from jawed vertebrates for ∼500 Ma. Recent chromosome-scale genomic analyses indicate that the common ancestor of modern cyclostomes likely shared the first of two GWD events with the extant gnathostome genomes but then diverged and underwent subsequent independent duplication events (possibly hexaploidization). Some of these duplications may have given rise to cyclostome-specific increases in receptor number (4649).

Remarkably, most human class II receptor genes cluster on chromosome bands 21q22, 6q23, and 1p36, whereas two are isolated on 11q23 and 3q22. Some of these gene locations are reflective of a human tetrad (21q22, 11q23, 1p36, 3q22), most probably resulting from two GWD rounds of a single prevertebrate locus of adhesion molecules related to gnathostome lymphocyte coreceptors involved in Ag presentation. An example of this could be on C. intestinalis chromosome 10 (50), where the two tunicate receptor chain genes map, thereby suggesting that IFN receptors may be part of this locus. Alignment of potential phosphorylation sites in C. intestinalis IFNAR1 and IFNGR1 cytoplasmic tails of IFNRA1, together with tandem location, suggest recent duplication independent of further GWD rounds.

Gene clustering and protein structural similarities may reveal evolutionary relationships. In the lamprey genome, eight of the receptor genes are organized in four pairs, GHR/TPOR (chromosome 35), PRLR/IL7RA (chromosome 44), IL12RB1/CRLF1 (chromosome 35), and IL12BC1/2 (chromosome 40), whereas six are organized in two triplets for IL6RA/IL11RA/CNTFRA (chromosome 1) and three IL10RA genes (undefined chromosomal region) (Table I). These clusters may reflect earlier tandem duplications. The group 1 receptors could have expanded first from early duplication of a common ancestor to give rise to a GHR/PRLR ancestor and CRLF3, followed by a local tandem duplication to GHR/TPOR, whereas the TPOR/EPOR duplication may have occurred later. Given its location, IL7RA may derive from a common ancestor with the group 1 receptor PRLR by tandem duplication, which is consistent with the prediction that IL-7 shows higher structural resemblance to long-chain cytokines (i.e., ligands for groups 1 and 2/3 receptors) rather than short ones (i.e., ligands for group 4/5 receptors) (17, 51, 52). However, in contrast to group 1 chains, which are all homodimer receptors, IL7RA, heterodimerizes with group 5 chains IL2RG or CRLF2 to form IL7 or TSLP receptors. This combination with a group 5 chain is a hallmark of group 4 receptor chains, such as IL4RA or IL3RB. On the basis of an ascendance from the group 1 receptor subfamily, we therefore hypothesize that IL7RA resembles the founder of group 4 receptor chains.

LPTR is related to ancestral Drosophila Dome and may have been generated by tandem duplication of a common ancestor with IL12RB2, followed by subsequent duplications resulting in IL12RB1, IL6ST, and LIFR. An IL12RB1-like common ancestor in turn may have given rise to CRLF1 by tandem duplication and further to IL12/27BC-like and IL6RA/-11RA/CNTFRA receptors (Supplemental Fig. 3). Thus, the combination of GWDs and tandem duplications likely contributed to most of the receptor gene expansion in lamprey.

We found that many receptor genes (i.e., IL13RA, CRLF2, IL4RA, IL12RB1, IL12RB2, GP130, LIFR, IFNAR1, IFNGR1, and IFNLR1) are single in the lamprey genome and are parts of clusters in Gnathostomata genomes (Fig. 2 and Supplemental Fig. 3). This indicates that tandem duplication has been a major mechanism for increasing receptor gene numbers in gnathostomes. Such duplications contributed to the generation of ligand/receptor diversity and fostered fine-tuned regulation of lymphoid/myeloid cell ontogeny. For example, expansion of the unique sea lamprey CRLF2 ancestral gene gave rise to the mammalian gene family for CRLF2, CSF2, IL3RA, and IL5RA, which encode α-chains for well-known CSF cytokines. This expanded family could have certainly impacted myeloid cell–type diversity (53). Also, the CRLF2 family expansion in elephant shark has generated six potential genes, one encoding a potential soluble form that may act either as a competing antagonist, as exemplified by IL13RA2, or as a soluble agonist, as exemplified by IL-12 (Supplemental Fig. 2). This scenario provides another potential regulatory level of cytokine signaling.

Identification of class I helical cytokine receptors offers an alternative strategy for defining cognate cytokines that are difficult to identify because of poor interspecies sequence conservation. Receptor chain combinations predicted from mammalian receptor–ligand pairs suggest the existence of 16 hypothetical cognate cytokines in the sea lamprey: three for group 1 ligands (GH, PRL, and TPO, CRLF3R being an orphan receptor), at least eight for groups 2 and 3 ligands (LPT, IL-6, IL-11, LIF, CNTF, IL-12, IL-27, and IL-35), at least five for groups 4 and 5 ligands (IL-4, IL-13?, CSF2, TSLP, IL-7), and at least three for class II ligands (IFN-α/γ, IL-10, IFN-λ) (Fig. 2) (1).

Furthermore, respective immune functions might be deduced on the basis of cytokine activities described in mammals, keeping in mind that “functional drift” during evolution has likely occurred. One could predict basic immune functions such as promotion of lymphocyte ontogeny by similarity to processes in gnathostomes. One could also posit the existence of a parallel pathway to that leading to the CD4 Th1/Th2 polarization (IL-12 and IL-4, IFN-γ and IL-10), even though the lamprey lacks MHC classes I and II molecules. These predicted activities are consistent with the existence of T-like cells expressing VLRAs or VLRCs and B-like lymphocytes expressing Abs VLRBs, respectively, and with a thymus-equivalent organ characterized in the gill folds (6, 10).

Some mammalian short helical cytokines show CSF activities (53), suggesting a parallel between the occurrence of novel cytokines and novel cell types/functions. For IL-5 (eosinophils), GM-CSF (granulocytes-macrophages, dendritic cells), and IL-3 (multiple lineages), cognate α-chain receptor genes colocalize with CRLF2/TSLP in the chondrichthyans (Fig. 2), suggesting that IL-3, GM-CSF, and IL-5 derive from a common ancestor with the unique CRLF2 gene found in the lamprey. Solely on the basis of sequence similarity, G-CSF (granulocytes) may derive from IL-6 duplication in a common ancestor with sharks. Similarly, EPO (erythrocytes) may be derived from a common ancestor with lamprey TPO (thrombocytes).

The number of class I and class II helical cytokine receptor chains encoded by the sea lamprey P. marinus genome and their relationship to gnathostome counterparts predicts the existence of 19 helical cytokines. Identification of these receptor chains and their potential combinations to form active receptors may provide an avenue for understanding their tissue expression profiles, identifying potential responding cells, isolating their cognate ligands, and subsequently detailing their role in the development and activation of WBCs. Compared with the higher number of cytokine receptors systematically found throughout Gnathostomata evolution, the smaller number in the lamprey may reveal a simpler cytokine network that regulates lymphocyte differentiation and polarization. The reduced complexity of the lamprey system relative to its gnathostome counterpart may thus encompass the essential elements of a vertebrate cytokine network and be a valuable guide for investigating basic questions of immune regulation as well as human pathologies.

This work is dedicated to the memory of William E. Paul (1936–2015), who coinitiated this project, for his vision and his unquenchable scientific curiosity (54). We thank Luigi Mariani for his support; Irene Salinas and Martin Flajnik for helpful suggestions; Jonathan Rast, Masayuki Hirano, Saby Das, John O’Shea, and Graham Le Gros for critical reading of the manuscript; and Elisa Casadei and Pedro Reche for fruitful discussions.

This work was supported by the Department of Surgery, University Hospital of Basel (J.-L.B.) and by National Institutes of Health Grants R01AI072435 and GM108838 (M.D.C.).

Max D. Cooper is a Distinguished Fellow of AAI.

The online version of this article contains supplemental material.

Abbreviations used in this article:

     
  • CNTFR

    ciliary neurotrophic factor receptor

  •  
  • EPO

    erythropoietin

  •  
  • EPOR

    erythropoietin receptor

  •  
  • GWD

    genome-wide duplication

  •  
  • HpoR

    hematopoietin receptor

  •  
  • LIFR

    leukemia-inhibiting factor receptor

  •  
  • LPTR

    leptin receptor

  •  
  • LRR

    leucin-rich repeat

  •  
  • NCBI

    National Center for Biotechnology Information

  •  
  • PRL

    prolactin

  •  
  • PRLR

    prolactin receptor

  •  
  • TPO

    thrombopoietin

  •  
  • TPOR

    thrombopoietin receptor

  •  
  • TSLP

    thymic stromal lymphoprotein

  •  
  • VLR

    variable lymphocyte receptor

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The authors have no financial conflicts of interest.

Supplementary data