Where Have All the KIRs Gone?
The killer-cell Ig-like receptors (KIRs) are encoded by a highly variable family of genes that has evolved extremely rapidly in primates. Four lineages of KIR have been identified, but the dominant lineage varies among primate species, and the KIR loci of smaller apes like gibbons have not been characterized. Abi-Rached et al. (p. 1379) therefore analyzed the KIR haplotypes of the two most divergent gibbon genera and found that their KIR genes did not play by the same rules as those of other primates. Four diverse haplotypes were identified, and all were unusually small. Although all four lineages of KIR were represented, the highly conserved gene KIR2DL4 was absent from two of the gibbon haplotypes and contained a premature stop codon in the others. This lack of functional KIR2DL4 correlated with an absence of a gene encoding MHC-G, the KIR2DL4 ligand. In addition, the gibbon haplotypes had few to no lineage III KIR and no functional gene encoding their products’ ligand, MHC-C. Through further sequence and phylogenetic analyses, the authors refined current models of KIR evolution and proposed a new composition of the ancestral KIR haplotype. Gibbon KIR haplotypes were subject to widespread deletion during evolution, resulting in several missing and hybrid KIR genes compared with haplotypes in other species. As lineage III expanded in these other species, activating KIR acquired an ability to recruit DAP12 instead of FcRγ as a signaling partner. This study presents extensive data advancing our understanding of KIR evolution and its coupling with the MHC locus.
Putting the Brakes on Macrophages
The bioactive lipid sphingosine 1-phosphate (S1P) exerts diverse biologic effects, including both induction and inhibition of cellular migration, through five different receptors (S1P1–5R). Because little is known about the negative regulation of macrophage migration, Michaud et al. (p. 1475) characterized the effects of S1P in these cells, which they found expressed S1P1R and S1P2R. S1P2R signaling slowed down macrophage chemotaxis, and mice deficient in S1P2R, but not in S1P1R, demonstrated increased peritoneal macrophage accumulation during peritonitis. S1P:SIP2R-mediated inhibition of chemotaxis was revealed to occur through SIP2R-induced cAMP generation. cAMP acted through protein kinase A to inhibit macrophage migration and chemoattractant-induced Akt phosphorylation. The phosphoinositide phosphatase PTEN, which negatively regulates PI3K signaling, has been proposed to mediate S1P2R's inhibitory effects in endothelial cells and fibroblasts but was not involved in S1P2R activity in macrophages. These data demonstrate that the S1P:S1P2R interaction may assist in the prevention of excessive macrophage recruitment to inflammatory sites.
Subdividing the Subsets
The transcription factor promyelocytic leukemia zinc finger (PLZF) is required for NKT cell effector functions. Alonzo et al. (p. 1268) investigated whether PLZF was also involved in the development or function of another innate-like T cell subset, γδ T cells. The authors found that PLZF was expressed in a phenotypically and functionally distinct subset of Vγ1.1+Vδ6.3+ T cells that was more likely to produce IL-4 and express CD4 than were its PLZF-negative counterparts. ThPOK, a transcription factor required for the maintenance of Th effector functions, was also expressed in the PLZF+ subset of γδ T cells and was important for their phenotype. This cell subset was diminished in mice lacking signaling lymphocyte activation molecule-associated protein but not in those lacking Fyn, indicating that PLZF+ γδ T cells had different developmental requirements than PLZF+ NKT cells. Interestingly, although γδ T cell lineage commitment requires strong TCR signals, disruptions in proximal TCR signaling led to dramatic expansion of PLZF+ Vγ1.1+Vδ6.3+ T cells. Mice deficient in Id3, which has been shown to enhance γδ T cell development, also had significantly more PLZF+ γδ T cells than wild-type mice. Taken together, these data identify two functional subsets of Vγ1.1+Vδ6.3+ T cells, of which the PLZF+ lineage develops in response to reduced TCR signal strength and a resulting decrease in Id3.
The Bacterium and the Podosome
Dendritic cells (DCs) express a variety of TLRs and other pattern recognition receptors and are important for the induction of immunity to bacterial infections. Because chronic infections are more commonly caused by Gram-positive than Gram-negative bacterial species, van Helden et al. (p. 1280) asked whether different intact pathogens might differentially affect DC activation. Indeed, Gram-negative bacteria were found to much more effectively induce DC maturation and migration than could Gram-positive organisms. Gram-negative bacteria also induced dissolution of podosomes, which allowed mature DCs to migrate at higher speeds than immature DCs. This podosome dissolution required TLR4 signaling, particularly pathways dependant on the adaptor protein TIR domain-containing adapter-inducing IFN-β. Following stimulation of DCs with Gram-negative bacteria, PGE2 production was upregulated, suggesting a link between TLR4 signaling and arachidonic acid metabolism that might be responsible for DC maturation and migration. These data reveal a mechanism by which Gram-negative bacteria induce DC maturation and suggest that the inability of Gram-positive bacteria to similarly affect DCs could contribute to the ability of these pathogens to escape immune attack and establish chronic infection.
At Nerve's End with Semaphorin
Semaphorins were originally identified as regulators of axon guidance during neuronal development but have since been shown to also play important roles in immunity. Semaphorin 4D (Sema4D) uses distinct receptors, Plexin-B1 and CD72, to mediate effects in the nervous and immune systems, respectively. However, it is not clear how Sema4D might modulate neuroinflammation, leading Okuno et al. (p. 1499) to analyze this molecule's effects on microglial activation. Sema4D enhanced microglial production of NO and inducible NO synthetase through Plexin-B1 and activation of ERK1/2. During experimental autoimmune encephalomyelitis (EAE) development, expression of both Sema4D and Plexin-B1 was upregulated in pathogenic lesions in the CNS. Plexin-B1–deficient mice, as well as mice specifically lacking Plexin-B1 in the CNS, demonstrated attenuated EAE development, supporting its role in neuroinflammation. Sema4D expression on T cells was important for EAE development, and blocking anti-Sema4D Abs inhibited neuroinflammation and disease progression. Thus, Sema4D on T cells interacts not with CD72, its previously identified immune regulatory ligand, but instead with Plexin-B1 on microglia to promote neuroinflammation and EAE pathogenesis. Blockade of Sema4D could be a valuable immunotherapeutic tactic to combat neuroinflammatory disease.
HIV’s Favorite Targets
It is unclear which CD4+ T cell subpopulations HIV might preferentially infect in vivo. Gosselin et al. (p. 1604) therefore divided primary human CD4+ T cells into subsets on the basis of chemokine receptor expression and assessed their relative permissiveness to HIV infection. Four subsets of memory T cells were analyzed: CXCR3+CCR6− (Th1), CCR4+CCR6− (Th2), CCR4+CCR6+ (Th17), and CXCR3+CCR6+ (Th1Th17). High levels of both CCR5-tropic (R5) and CXCR4-tropic (X4) HIV replication and integration were observed in CCR6+ Th17 and Th1Th17 cells but not in CCR6− Th1 cells. CCR6− Th2 cells were also permissive to X4 but not to R5 HIV infection. Extending these results to in vivo situations, the authors analyzed CD4+ T cell subsets in HIV-infected individuals. In these individuals, CCR6+ T cells showed higher levels of infection than CCR6− T cells, and this correlated with a decrease in the frequency of peripheral blood CCR6+ T cells, even in individuals with effectively controlled viral replication. Analysis of the cytokine profiles expressed by the CCR6+ subsets suggested that upregulation of TNF-α and the CCR6 ligand CCL20 could promote HIV replication and the attraction of additional CCR6+ cells, leading to further amplification of HIV replication. This detailed study reveals the relative susceptibility of CD4+ T cell subsets to HIV infection and will be useful to future research into HIV pathogenesis and treatment.
The source and identity of intratumoral DCs, which may facilitate tumor immunoescape, have been unclear. Diao et al. (p. 1261) have found that conventional DCs (cDCs), which are normally found in lymphoid tissue, develop within tumors from the same pre-cDC population that differentiates into splenic cDCs. These pre-cDCs were identified in tumors by surface markers, morphology, and replicative potential comparable with those of pre-cDCs found in the spleen and bone marrow. Migration of pre-cDCs from the blood into the tumor was mediated by CCL3, which was previously reported to induce pre-cDC chemotaxis. Once in the tumor, pre-cDCs were able to differentiate into cDCs with proliferative potential. Although it has been suggested that intratumoral DCs demonstrate defects in Ag presentation, these intratumoral cDCs were able to induce similar levels of Ag-specific T cell proliferation as could splenic cDCs. Thus, pre-cDCs are recruited to tumors via CCL3, where they mature into competent cDCs. This description of the source and functional capacity of intratumoral cDCs will be important for the understanding of DC activity in tumors and may suggest novel routes for immunotherapy.
Giving Burkholderia the Boot
Burkholderia mallei is a Gram-negative pathogen that causes severe pneumonic infections in many parts of the world and has potential for use as a bioweapon. Little is known regarding innate immune responses that might control infection with this organism, necessitating a better understanding of its pathogenesis. Goodyear et al. (p. 1445) therefore assessed the roles of MCP-1 and monocytes in a mouse model of B. mallei infection. Compared with wild-type mice, mice deficient in either MCP-1 or its receptor, CCR2, were significantly more susceptible to B. mallei infection and demonstrated increased bacterial burden and inflammatory lesions in the lung and liver. CCR2−/− mice also had significantly reduced monocyte and DC infiltration into the airways postinfection. Depletion of monocytes from wild-type mice increased mortality and bacterial burden following B. mallei infection, indicating that monocytes were important for controlling infection and preventing bacterial dissemination from the lung. Analysis of cytokine responses in infected MCP-1−/− and CCR2−/− mice demonstrated a severe impairment in IFN-γ production, and treatment of CCR2−/− mice with exogenous IFN-γ protected them from B. mallei-induced lethality. Thus, in B. mallei infection, MCP-1 regulates monocyte responses and IFN-γ production and thereby protects against lethal pneumonia.
Summaries written by Jennifer Hartt Meyers, Ph.D.