Polysialic Acid, a Glycan with Highly Restricted Expression, Is Found on Human and Murine Leukocytes and Modulates Immune Responses¹

Polysialic acid (polySia)³ is an unusual glycan by almost any definition. Structurally, it comprises repeating sialic acid monomers with α2,8 linkages. Its size in terms of monosaccharide units, or degree of polymerization (DP), is large, with reported values of 50–150 ranging up to >370 residues (1, 2). This contrasts with the typical N-linked glycan containing 10–12 monomers, and is similar to glycosaminoglycans (GAGs), which average 80–100 residues (3). The synthesis of polySia is also unusual: an entire chain is produced by a single enzyme acting on classical N-linked and, less commonly, O-linked core structures. Two polysialyltransferases, ST8Sia IV (PST) and ST8Sia II (STX), with distinct expression patterns are involved in synthesis of polySia. In contrast, the synthesis of most glycans requires the coordinated action of many enzymes. Therefore, while glycan structure is difficult to track genetically, polySia can be localized by the expression patterns of ST8Sia IV and ST8Sia II. Furthermore, while many glycan structures can modify any number of protein cores, the polysialyltransferases appear to be highly selective in their scaffold choices. Aside from autopolysialylation of the ST8Sia IV and ST8Sia II enzymes, only four other protein carriers have been identified: the neural cell adhesion molecule (NCAM, also termed CD56), the α-subunit of the voltage-gated sodium channel, CD36, and neuropilin (4, 5, 6, 7). Of these, NCAM is by far the most commonly used scaffold.

Polysialylated NCAM is prominent in the developing nervous system, where it has been most extensively studied. A large body of work has shown that polySia affects neuronal functions as varied as migration (8, 9), cytokine response (9, 10), and cell contact-dependent differentiation (11). Provocatively, these same functions are vital components of immune function. Leukocytes migrate throughout the body, guided by specialized cytokines, termed chemokines, to effect both homeostatic and inflammatory functions that are often dictated through cytokine and cell contact-dependent signals.

Intriguingly, expression of both ST8Sia IV and ST8Sia II has been documented in the immune system, suggesting that polySia is abundant therein. In the adult human, primary and secondary lymphoid organs including the placenta, spleen, thymus, intestine, and peripheral blood express ST8Sia IV, while ST8Sia II is produced in the thymus (12). Our recent work in the mouse suggests that polySia plays an important role in progenitor trafficking to the thymus (P. M. Drake, manuscript submitted). Furthermore, NCAM (CD56) is expressed on two subsets of mature human lymphocytes: NK cells and NKT cells. Although NCAM has long been used as a marker for these cell populations (13), its functional role remains undefined. We postulated that in the immune system NCAM functions as a scaffold for presentation of polySia, and that the glycan itself confers function to this glycoprotein. Moreover, the functional significance of these observations is unknown.

The documented role of polySia in modulating cell adhesion, migration, and cytokine response in the nervous system motivated us to investigate the possibility of analogous functions in the immune system. Herein, we describe polySia expression on NCAM in human NK cells, as well as mouse hematopoietic progenitors and myeloid cells. In support of a role for polySia in the immune system, we demonstrate that human NK cells regulate the expression and length of polySia with activation state, and that ST8Sia IV^−/− mice have aberrant contact hypersensitivity responses and enhanced tumor growth as compared with wild-type mice.

Wild-type C57BL/6 and congenic GFP⁺ mice were purchased from The Jackson Laboratory. Mice were housed in specific pathogen-free conditions. Experiments were approved by the University of California at Berkeley’s Animal Care and Use Committee. The generation of ST8Sia IV^−/− and ST8Sia II^−/− mice has been described (14, 15).

Human leukocytes were obtained as buffy coats from the American Red Cross, Oakland, CA. PBMCs were prepared as described (16). NK cells were isolated using a magnetic bead-based method (Dynal NK cell negative isolation kit, Invitrogen). Cells were either lysed directly or cultured in RPMI 1640 with 10% FBS. Endoneuraminidase N (Endo N, 1/1000 to 1/2000 dilution) and/or 6000 U/ml recombinant IL-2 (National Cancer Institute Preclinical Repository) were added as indicated.

Human fetal bone marrow was obtained from elective pregnancy terminations at the University of California San Francisco with approval of the Committee for Human Research. Bone marrow was harvested from long bones as previously described (17). Light-density cells were isolated by centrifugation on a layer of 1.077 g/ml NycoPrep (Axis-Shield). Both total and light-density cell fractions were studied.

Human NK cell cultures were pelleted, flash frozen, and stored at −80°C until analysis. Pellets from five individual donors were combined, and the degree of polymerization was determined as described (2).

Abs used for flow cytometry are as follows. From eBioscience: fluorescein-conjugated anti-CD3ε (145-2C11), anti-CD4 (GK1.5), anti-CD11b (M1/70), anti-CD25 (PC61.5), anti-TCRβ (H57–597), anti-Gr-1 (RB6–8C5), and anti-TER119; PE-Cy5-conjugated Sca-1 (D7), and isotype control rat IgG2a; and allophycocyanin-conjugated secondary anti-mouse IgG. From BD Biosciences: purified anti-polySia (12F8), anti-NCAM (NCAM-13), and rat IgM isotype control; fluorescein-conjugated anti-CD8a (53-6.7) anti-TCRγδ (GL3), anti-NK1.1 (PK136), anti-B220 (RA3-6B2), and isotype controls mouse IgG2a (G155-178), rat IgG1 (R3-34), rat IgG2a (R35-95), and rat IgG2b (A95-1); PE-conjugated anti-CD8a (53-6.7), anti-CD3 (SK7), anti-CD33 (Leu-M9), anti-CD81 (JS-81), anti-CD117 (104D2), anti-CD25 (PC61), and anti-CD117 (2B8) and isotype controls rat IgG1 (R3-34) and rat IgG2a (R35-95); PE-Cy5-conjugated anti-CD3 (17A2), anti-CD44 (IM7). From Jackson ImmunoResearch Laboratories: allophycocyanin-conjugated secondary anti-rat IgM. From Invitrogen: fluorescein-conjugated anti-CD3 (S4.1), anti-CD15 (V1MC6), anti-CD34 (581), and isotype control mouse IgM; PE-conjugated anti-CD7 (CD7-6B7), anti-CD34 (581), anti-CD45 (HI30), anti-CD105 (SN6), and isotype controls mouse IgG1, mouse IgG2a, and mouse IgG2b. From Beckman Coulter: PE-conjugated anti-CD2 (SFCI3Pt2H9-T11). From Exalpha Biologicals: fluorescein-conjugated anti-CD56 (C5.9). The production of mAb 735 has been described (18).

Cells were isolated and immediately incubated for 10 min with mouse BD Fc block (anti-FcγIII/IIR; BD Biosciences), followed by the addition of Abs for staining. After 20 min cells were washed twice in PBS and analyzed on a FACSCalibur (BD Biosciences) using CellQuest (BD Biosciences) software. Hematopoietic stem cells were defined as Lin⁻ (including TER119, CD3, CD4, CD8, B220, NK1.1, Gr-1, TCRβ, TCRγδ, CD11b), c-Kit⁺, Sca-1⁺. Human cells were analyzed on an LSR II flow cytometer (BD Biosciences). Metaanalyses were performed using FlowJo software (Tree Star). Differences between controls, which were stained with an irrelevant Ab of the same isotype, and the experimentals, which expressed polySia, were calculated using the population comparison function of FlowJo software. For sorted cells, freshly isolated mouse bone marrow was centrifuged over a layer of Ficoll-Paque Plus (Amersham Biosciences), and then light-density cells were labeled with anti-c-Kit (PE) and purified anti-polySia (12F8) followed by anti-rat IgM (allophycocyanin). Desired cell subsets were sorted using a Dako-Cytomation MoFlo high-speed sorter (Dako) or a FACSAria (BD Biosciences).

Erythroid progenitor cultures were initiated in septuplicate with 2.5 × 10² to 2.0 × 10³ cells/dish. Colony-forming cell assays were performed as previously described (19) in serum-deprived medium with substitution of 10% FBS for low-density lipoprotein (HyClone Laboratories). Growth was supported by 100 ng/ml recombinant rat stem cell factor (Amgen) and 10 U/ml recombinant human (rh) erythropoietin (Amgen). After 7 days, cultures were scored for burst-forming units erythroid identified as distinct clusters of erythroid cells with or without myeloid cells and myeloid colony-forming cells having a dispersed cell morphology. Myeloid progenitors were assayed as previously described (20). Sorted cells were cultured, in triplicate, at 1.0 × 10² to 2.0 × 10³ cells/dish in the described medium containing 100 ng/ml recombinant rat stem cell factor, 20 ng/ml rhIL-6, 20 ng/ml recombinant murine IL-3, 20 ng/ml rhM-CSF (R&D Systems), and 20 ng/ml rhG-CSF (Amgen). After 7 days, cultures were scored visually for colonies (50+ cells) and clusters (10–49 cells). The erythroid and myeloid assays were repeated twice with similar results.

OP9 cells transfected with Delta-like-1 (DL1) and GFP, or with GFP alone, were kindly provided by Dr. Juan Carlos Zúñiga-Pflücker (Sunnybrook & Women’s Research Institute). OP9-GFP and OP9-DL1 cells were cultured and passaged as described (21). Sorted bone marrow subsets were plated onto a semiconfluent layer of either OP9-GFP or OP9-DL1 cells in complete DMEM-10 with 1 ng/ml IL-7 (R&D Systems) and 5 ng/ml Flt-3 ligand (R&D Systems). Cocultures were maintained, passaged as described (21), and harvested at 18 days. Cells were counted with a hemocytometer and analyzed by flow cytometry for expression of lineage markers. The experiment was repeated three times with similar results.

Wild-type mice (6–8 wk) received a single i.p. dose of either 5-FU (150 mg/kg) or vehicle. Following drug administration, bone marrow was harvested from two to three mice at each time point (days 1, 2, 7, 10, and 12) for flow cytometric analyses. The experiment was repeated three times with similar results.

Wild-type mice (6–8 wk) were injected i.p. once a day for 5 days with 100 μl of either vehicle (PBS + 2% BSA) or vehicle containing 2 μg G-CSF. Mice, two to three per group, were sacrificed after 5 days. Bone marrow was harvested for analysis by flow cytometry. The experiment was repeated three times with similar results.

Freshly isolated mouse bone marrow and brain samples were disrupted on ice in 20 mM Tris-Cl (pH 8.0), 140 mM NaCl, 10% glycerol, 1% Nonidet P-40, 2 mM EDTA, 10 mM NaF, and proteinase inhibitor cocktail (Calbiochem). Lysates were separated on a 3–8% Tris-acetate gel (Bio-Rad), transferred to nitrocellulose (Bio-Rad), and nonspecific reactivity was blocked by incubating the blots for 2 h in 5% milk in PBS with 0.05% Tween 20 (PBST). Blots were then incubated overnight at 4°C in PBST with primary Ab at 1/5000 (anti-polySia, mAb 735) or 1/500 (anti-NCAM, mAb NCAM-13). After washing three times for 5 min in PBST, blots were incubated for 90 min at room temperature in PBST with HRP-conjugated secondary Abs (Jackson ImmunoResearch Laboratories) at 1/5000. Blots were washed three times for 5 min in PBST and signal was detected using SuperSignal West Pico chemiluminescence substrate (Pierce).

Bone marrow lysates were prepared as described above from 10 wild-type and 10 ST8Sia IV^−/− mice, and precleared by a 30-min incubation at 4°C with protein G-Sepharose beads (Invitrogen). Ab-conjugated beads were prepared by incubating 5 μg mAb 735 with 50 μl protein G-Sepharose beads at 4°C. mAb 735-conjugated beads were added to lysates and tumbled for 1 h at 4°C. Following incubation, beads were washed extensively in ice-cold wash buffer [0.1% Triton X-100, 50 mM Tris-Cl (pH 7.4), 300 mM NaCl, 5 mM EDTA, 0.02% NaN₃ (w/v)] and then in ice-cold PBS. PolySia was removed from captured proteins by direct treatment with 2 μl of Endo N (3 h at 37°C). Then, the sample was boiled in sample buffer (XT sample buffer, XT reducing agent; Bio-Rad) and run on a 3–8% Tris-acetate gel (Bio-Rad). Bands were visualized with a mass spectrometry-compatible silver stain (Silver Quest; Invitrogen).

The preparation and analysis of samples were performed at the Taplin Biological Mass Spectrometry Facility, Harvard Medical School. Silver-stained bands were excised and digested with trypsin, and analyzed by ESI-LC-MS/MS on an LTQ linear ion-trap mass spectrometer (ThermoFisher Scientific). Peptide sequences (and hence protein identity) were determined by matching protein databases with the acquired fragmentation pattern by the software program Sequest (ThermoFisher Scientific) (22). Spectral matches were manually examined and multiple identified peptides per protein were required.

ST8Sia IV^−/− mice were crossed with congenic wild-type mice expressing GFP (stock 004353, The Jackson Laboratory). F₁ progeny were back-crossed with ST8Sia IV^−/− animals to yield ST8Sia IV^−/−;GFP^+/− mice, which were used as bone marrow donors for these studies. Genotype was confirmed by PCR as previously described (14). Sorted cell populations were mixed with 1 × 10⁶ whole wild-type bone marrow cells for injection. Wild-type recipient mice (three per donor subset) were irradiated with one dose of 900 rad and injected i.v. with prepared cells. After 21 days, recipients were sacrificed and their lymphoid organs analyzed for GFP⁺ cells. The experiment was repeated twice with similar results.

2,4-Dinitrofluorobenzene (DNFB) was used to induce CHS in wild-type or ST8Sia IV^−/− mice (3–10 mice per group). Animals were sensitized by painting bare abdominal skin on 2 consecutive days with 20 μl of 0.5% DNFB in acetone/olive oil (4/1). Mice were challenged 7 days later with an application of 20 μl of 0.5% DNFB on the right ear, and vehicle alone on the left ear. The inflammatory response was assessed by measuring ear thickness with digital calipers at 24, 48, and 72 h. In some cases, animals were euthanized and their ears removed for histological evaluation. The experiment was repeated four times with similar results.

Mouse RMA and RMA-S (which have reduced MHC class I expression) NKT cell tumor cell lines were obtained from the American Type Culture Collection. Each experimental group (wild-type and ST8Sia IV^−/−) contained 5–10 mice (4–12 wk) that were age-matched within 2 wk. RMA or RMA-S cells (10⁴-10⁵) were injected s.c. into the flank. Palpable tumors were measured daily with digital calipers. Mice were euthanized after loss of >20% of original body weight, when a tumor reached >1.5 cm or became ulcerated. Animals surviving 60 days were considered tumor-free. The experiment was repeated twice with similar results.

The Univeristy of California Davis Comparative Pathology Laboratory performed complete blood counts, which were validated by visual examination of blood smears. Histological evaluation of CHS ear specimens was also performed.

Unless otherwise stated, numbers represent the means ± 1 SD.

Statistical significance in tumor assays was calculated using a two-tailed Mann-Whitney U test. Statistical significance of the remaining data was calculated using an unpaired two-tailed Student’s t test. p < 0.05 was considered significant.

Flow cytometric analyses showed that human PBMCs expressed the NCAM protein scaffold and its polysialic acid modifications, which were sensitive to the polySia-specific neuraminidase Endo N (Fig. 1,A). Upon activation with IL-2, the cell-surface levels of both the underlying protein and the attached glycan increased (Fig. 1 B; n = 10). To address whether the observed increase in polySia expression reflected differential proliferation of CD56^dim and CD56^bright NK cell populations during the assay, sorted cells with these phenotypic characteristics were analyzed in parallel; similar responses were observed (data not shown). These data suggest that polySia levels are regulated by NK cell activation.

As the DP of polySia on human primary cells has not been explored, purified NK cells were cultured with or without IL-2 for 48 h. Samples from five individual donors were combined for analysis. Intact polySia chains on glycan cores containing lactosamine were released by endo-β-galactosidase treatment. This method may have neglected to free polySia that was attached to NCAM through alternate core structures; however, the relative amounts of total sialic acid released from resting and activated NK cells were in accord with our flow cytometry (Fig. 1) and immunoblotting data (not shown). The liberated polySia was analyzed by HPLC as previously described to determine chain length (2). Total sialic acid content [as N-acetyl neuraminic acid (NANA)] of HPLC fractions was monitored to track the relative abundance of each DP population. In both resting and activated NK cells, a large fraction of total NANA was associated with small non-polySia glycans (DP 1–10), most likely reflecting the capping groups of typical N- and O-linked glycans. Plotting percentage of total NANA vs DP showed that activation of NK cells increased the abundance of NANA associated with polySia (Fig. 2). Additionally, chain length, which was extremely heterogeneous on resting cells, was compressed into the middle range (DP 11–140) upon activation.

To analyze the potential immunological properties of polySia, we used a mouse model. First, we characterized the expression patterns of polySia on immune subsets in wild-type animals. In contrast to their human counterparts, polySia was not detectable on mouse NK cells (Fig. 3,C). This finding was consistent with RT-PCR analyses on sorted NK cells that revealed an absence of NCAM and ST8Sia IV (data not shown). Interestingly, robust polySia expression was detected on wild-type mouse bone marrow subsets (Fig. 3 A). This expression was conserved in ST8Sia II^−/− mice but absent in ST8Sia IV^−/− mice, indicating that the latter enzyme was responsible for polySia on these cells.

Bone marrow polySia expression correlated with receptor tyrosine kinase c-Kit expression, suggesting that the polySia⁺ cells were hematopoietic progenitors. We defined four subsets according to their relative levels of polySia (PSA) and c-Kit (Kit) (PSA^neg/Kit^high, PSA^low/Kit^high, PSA^high/Kit^high, PSA^low/Kit^low; Fig. 3,A). The populations comprised 12%, 6%, 12%, and 42%, respectively, of total c-Kit⁺ bone marrow cells. Extensive phenotypic analyses by flow cytometry suggested that these subsets comprised cells that were differentiating along a myeloid pathway (Table I). The PSA^neg/Kit^high population included hematopoietic stem cells (defined as Lin⁻, c-Kit⁺, Sca-1⁺) and most appeared to be progenitors as they were Lin⁻ and expressed CD34. The second population (PSA^low/Kit^high) comprised more committed progenitors with near uniform expression of CD34, and to a lesser extent CD11b and Gr-1. As differentiation proceeded, evidenced by a reduction in CD34 expression and an increase in CD11b and Gr-1 expression, PSA levels dramatically increased in parallel (PSA^high/Kit^high). The fully differentiated progeny (CD34^+/−, CD11b⁺, Gr-1^high) had polySia levels that were comparable to the progenitors (PSA^low/Kit^low). Consistent with the latter population containing mature myeloid cells, we found low levels of polySia expression on peripheral wild-type, but not ST8Sia IV^−/−, Gr-1⁺ splenocytes (Fig. 3 B). Thus, myeloid differentiation is characterized by a wave of high levels of polySia expression. In contrast to the mouse, human fetal bone marrow did not contain these polySia⁺ myeloid populations, and human peripheral myeloid cells did not express polySia (data not shown). The polySia⁺ subset in human fetal bone marrow consisted of NK cells, as determined by the phenotype: CD56⁺, CD7⁺, CD33⁻, and CD34⁻ (data not shown). Note that the observed differences between adult mouse bone marrow and fetal human bone marrow may be attributable to variations in polySia expression during development.

To confirm the phenotypic analyses, the bone marrow populations defined by polySia and c-Kit were sorted by flow cytometry and tested in vitro and in vivo for their ability to give rise to various immune lineages. For these experiments, the PSA^neg/Kit^high subset served as a positive control, as hematopoietic stem cells were contained in this population (data not shown). In colony-forming assays testing erythroid and myeloid potential, both the positive control and the progenitor subset, PSA^low/Kit^high, gave rise to these lineages, forming both erythroid blasts and colonies, as well as myeloid colonies and clusters (Figs. 4). Of the more differentiated populations, the immature myeloid cells, PSA^high/Kit^high, showed an intermediate ability to form myeloid clusters and did not produce erythroid populations. The fully differentiated PSA^low/Kit^low cells did not produce colonies in either assay.

To test lymphoid progenitor potential, sorted bone marrow subsets were cocultured with OP9 cells transfected with a GFP vector control, or with GFP and the Notch ligand, DL1. In this assay, multipotent progenitors develop into B cells when cultured on OP9-GFP feeders, while DL1 induces T cell development. NK cells develop at lower efficiencies under both conditions. In accord with the phenotypic data and colony-forming assays, the positive control, PSA^neg/Kit^high, and the progenitor population, PSA^low/Kit^high, produced all the lymphoid lineages, the latter with a 10-fold reduction in efficiency as compared with the former (Fig. 5). The immature myeloid subset, PSA^high/Kit^high, did not produce NK or T cells and generated almost 1000-fold fewer B cells than the positive control population. The mature myeloid population, PSA^low/Kit^low, did not proliferate.

For in vivo experiments, GFP⁺ congenic wild-type mice were used as donors so that engrafted lineages could be identified using this fluorescent reporter. Flow-sorted bone marrow subsets (∼28,000 cells/mouse) were injected with a survival dose (1 × 10⁶ cells/mouse) of wild-type bone marrow into irradiated wild-type recipient animals. Three weeks later mice were sacrificed and organs analyzed for GFP⁺ populations (Table II). Both the positive control, PSA^neg/Kit^high, and the progenitor population, PSA^low/Kit^high, gave rise to erythroid (TER119⁺), myeloid (Gr-1⁺ and CD14⁺), and lymphoid (DX5⁺ and B220⁺) cells in the bone marrow and spleen. No statistically significant differences were noted in the numbers of cells recovered from these two donor populations. In contrast, the numbers of GFP⁺ cells isolated from the immature myeloid subset, PSA^high/Kit^high, and the mature myeloid population, PSA^low/Kit^low, were reduced by an average of ∼40-fold and ∼500-fold, respectively, as compared with the progenitor subsets.

Next, we asked whether development of the four bone marrow subsets was temporally linked. Mice were treated with one bolus of 5-FU to deplete cycling cells, and the disappearance and reappearance of the polySia/c-Kit-defined populations was followed over 12 days. As expected, the depletion and recovery of the progenitors preceded by 1 or 2 days that of the mature subsets (Fig. 6).

Finally, we stimulated myeloid development and characterized the response of the bone marrow subsets. Briefly, wild-type mice were injected every 24 h with G-CSF for 5 days to induce expansion of the myeloid compartment, and bone marrow was analyzed by flow cytometry. In accord with the preceding phenotypic in vitro and in vivo data, both the progenitor population (PSA^low/Kit^high) and the immature and mature myeloid subsets (PSA^high/Kit^high and PSA^low/Kit^low) significantly expanded in response to G-CSF treatment as compared with vehicle-treated control mice (p < 0.01; Fig. 7). Collectively, the data in Table II and Figs. 3–7 reveal that polySia expression is expressed and modulated during myeloid development in the mouse.

Given the expression of polySia on hematopoietic progenitors and myeloid cells, we asked whether there were obvious changes in the distribution of immune subsets in ST8Sia IV^−/− as compared with wild-type mice. Hematological analyses revealed minimal differences: a statistically significant, slight increase in the percentage of lymphocytes in peripheral blood, and a corresponding although not significant decrease in the percentage of circulating monocytes and neutrophils (data not shown). No changes in the numbers or phenotype of peripheral myeloid lineages were noted (data not shown).

As this study was the first description of polySia on myeloid cells, the underlying protein scaffold was unknown. Candidate proteins that are expressed by other cell types that carry polySia modifications include NCAM and CD36. To identify the scaffold on the myeloid subsets that were the subject of this investigation, we immunoprecipitated wild-type and ST8Sia IV^−/− bone marrow lysates with an anti-polySia Ab, treated the precipitatate with Endo N to remove polySia, and separated the deglycosylated proteins by SDS-PAGE. Following electrophoresis, two silver stained bands of ∼120 and 140 kDa were observed in the wild-type, but not in the ST8Sia IV^−/−, samples (Fig. 8,A). The bands were excised and analyzed by electrospray mass spectrometry (LTQ linear ion trap), which identified both bands, on the basis of two peptides each, as NCAM (data not shown). In accord with this finding, both flow cytometry and immunoblotting confirmed the presence of NCAM on the relevant cells in mouse bone marrow (Fig. 8, B and C).

Next we tested immune responses in ST8Sia IV^−/− mice. First, we used a CHS assay. Wild-type and ST8Sia IV^−/− mice were sensitized with the hapten DNFB; a week later, they were challenged with an application of DNFB to one ear, and vehicle alone to the other ear. Wild-type mice responded as expected, with peak swelling observed around 24 h (23). Interestingly, the ST8Sia IV^−/− response equaled or exceeded the wild-type response at 24 h, and inflammation continued to increase through 48 and 72 h. Fig. 9 A shows the results of a representative experiment in which ST8Sia IV^−/− ear thickness was statistically increased at all time points as compared with the response of wild-type animals.

We also assessed the CHS response at 72 h on a histological level and noted significantly more inflammation and edema after DNFB treatment of ST8Sia IV^−/− mice as compared with controls, whose lesions were much less severe (Fig. 9 B). Injury to the epithelium of ST8Sia IV^−/− ears was also more pronounced, with diffuse epidermal hyperplasia, intercellular edema, and ulceration. The ST8Sia IV^−/− vehicle-treated ears also demonstrated minimal to mild edema whereas the wild-type controls had no significant lesions.

We used immunosensitive and immunoresistant cell lines to test the response of ST8Sia IV^−/− mice to a tumor challenge. RMA-S cells, which have reduced MHC class I expression, are sensitive to NK killing, while the parental RMA cells form tumors in wild-type animals (24). In initial experiments, RMA-S cells formed tumor masses in NK cell-depleted wild-type animals (25), but not wild-type or ST8Sia IV^−/− mice. This finding suggested that the NK cell compartment of ST8Sia IV^−/− mice was intact (data not shown). In contrast, injection of RMA cells into wild-type, ST8Sia IV^−/−, or the immunodeficient TCRβ^−/− mice led to uncontrolled tumor growth in all cohorts, requiring euthanasia of the animals when their tumors exceeded acceptable size limitations (Fig. 10). Importantly, tumor growth in ST8Sia IV^−/− mice was significantly faster than in wild-type mice, and was comparable to the rate observed in TCRβ^−/− mice (p < 0.02).

PolySia expression and function have been extensively studied in the nervous system, but the distribution and role of polySia in the immune compartment is still undefined. Herein we demonstrated the presence of polySia on human NK cells and its absence on other immune subsets, a finding that is in accord with two earlier reports (26, 27). NK cells up-regulated the expression of polySia and its scaffold NCAM upon activation by IL-2 treatment. This coregulation suggested that the heightened polySia signals were due to an up-regulation of both scaffold and glycan, rather than solely attributable to expanded polySia chain length. In support of this conclusion, a comparison of DP on resting and activated cells showed that upon activation total polySia was increased and the DP became more homogenous, tending toward small- to medium-length chains. To our knowledge, this is the first elucidation of polySia DP on primary human cells. The DP values we observed were slightly larger than analogous structures that were isolated from cell lines and chick brain (1, 28). Although this distinction may be due to real differences among samples, it more likely reflects the fact that our method protects samples from acid-catalyzed internal hydrolysis. Importantly, this result shows that the size of polySia chains on human immune cells is comparable to that of GAGs, polysaccharides that play critical structural and functional roles in the extracellular matrix compartment. The repeating negative charge present on polySia is also reminiscent of highly anionic GAGs, and this characteristic is likely critical to its function.

Mouse NK cells did not express NCAM or polySia; however, both were coexpressed in mouse bone marrow. Immunoprecipitation of bone marrow lysates with anti-polySia Abs and identification of the resulting bands by mass spectrometry demonstrated that NCAM was the underlying scaffold. Analyses of ST8Sia II^−/− and ST8Sia IV^−/− mice indicated that the latter enzyme produced the glycan.

Flow cytometry revealed four bone marrow subsets based on expression of polySia and c-Kit. These populations (PSA^neg/Kit^high, PSA^low/Kit^high, PSA^high/Kit^high, PSA^low/Kit^low) bore cell surface Ags that were characteristic of myeloid differentiation. The PSA^neg/Kit^high subset contained hematopoeitic progenitors, the PSA^low/Kit^high population, multipotent progenitors, and the PSA^high/Kit^high, PSA^low/Kit^low groups, immature and mature myeloid cells, respectively. These findings were confirmed by in vitro and in vivo functional studies. 5-FU experiments demonstrated that the developmental kinetics of the four populations were temporally linked, and thus these populations shared a common lineage. Finally, the progenitor population, PSA^low/Kit^high, and the immature and mature myeloid subsets, PSA^high/Kit^high and PSA^low/Kit^low, respectively, all expanded in response to treatment with G-CSF, further evidence that these populations are myeloid.

We observed a dramatic different in CHS response in ST8Sia IV^−/−, which sustained augmented inflammation, as compared with wild-type. This striking observation elicits some interesting hypotheses. Considering possible roles for polySia in the immune system, we noted that both NK and myeloid cells are cytotoxic populations, carrying small positively charged antimicrobial peptides (AMPs) (29, 30, 31). Negatively charged GAGs such as heparan sulfate can bind and neutralize AMPs (32, 33), and thus it is possible that polySia with its structural similarities to GAGs has parallel activities. Expression of polySia on cytotoxic cells may localize AMPs released from leukocytes to prevent damage of surrounding tissues. The excessive CHS inflammatory response noted in ST8Sia IV^−/− mice is consistent with this notion, as granulocytes that were initially drawn to the site might have injured tissues by uncontrolled AMP leakage. In turn, damaged tissue would release proinflammatory signals, upregulating the immune response (34). This cycle is consistent with the protracted inflammatory reaction, the increased tissue damage, and the elevated leukocytic infiltrate noted in ST8Sia IV^−/− DNFB-treated ears relative to wild-type.

Additionally, GAGs, which bind cytokines and chemokines, modulate their availability and local concentration by either presenting or sequestering these molecules. It is possible that polySia plays a similar role. Studies in the nervous system have already demonstrated that neuronal responses to the cytokines brain-derived neurotrophic factor (BDNF) and platelet-derived growth factor (PDGF) are affected by polySia expression, although the mechanism has not been elucidated (9, 10). As the development and function of the immune system depend heavily on the influence of cytokines, polySia could have a great impact. However, its effects would likely be complex since cytokines that regulate signaling networks that, in turn, direct fate decisions, trafficking, and activation might be involved. The decreased immune response to ectopic tumors noted in ST8Sia IV^−/− mice raises the intriguing possibility that these processes are influenced by polySia.

To our knowledge, this is the first demonstration of either polySia or NCAM expression on myeloid cells. The discovery of such expression in the mouse provides an opportunity to manipulate and study the immunological role of these molecules in a biological context, including in vivo models. The findings could have important implications for human health, as polySia and NCAM are not only expressed on human NK cells but also decorate the surfaces of a variety of tumors and bacteria. For instance, polysialylated NCAM, which is found on a number of cancers including gliomas, small-cell lung carcinomas, and Wilms’ tumors, is positively associated with metastasis and disease progression (35, 36, 37). We postulate that tumors co-opt the immune system’s strategy of using polySia to modulate responses to chemokines and growth factors, thus gaining a competitive advantage. Regarding bacteria, polySia expression is correlated with the virulence of pathogenic strains of Escherichia coli K1 and group B Neisseria meningitidis (38, 39). It is possible that bacterial polySia binds and neutralizes AMPs, sequestering them at a safe distance from the cell wall. Similarly, a subset of chemokines also has antibiotic activity (40), and polySia, which has been shown to modulate chemokine functions (9, 10), may protect bacteria from this threat. Collectively, our data point to a role for polySia in the complex processes involved in immunological development and host defense.

The authors have no financial conflicts of interest.