Role of the Orphan Receptors, LAIR-1,Toso and CD300, in Regulating Inflammation

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To study the in vivo function of LAIR-1, we generated LAIR-1-/- and LAIR-1flox/flox mice on a C57Bl/6 background. Although LAIR-1-/- mice are healthy and fertile, and have normal longevity in pathogen free environment, they do show certain phenotypic characteristics distinct from wild-type (wt) mice. They develop an increased percentage of splenic B cells and dendritic cells, along with decreased levels of T cells, of which a higher frequency are CD4+CD25+. As the LAIR-1-/- mice age, the splenic T cell population shows a higher frequency of activated T cells and a higher percentage of effector/memory T cells. Since LAIR-1+/+ and LAIR-1-/- T cells traffic with equal proficency to peripheral lymphoid organs, this is probably not the result of abnormal T lymphocyte trafficking. Functional experiments showed that antigen (Ag) specific LAIR-1-/- CD4 T cells proliferate significantly less than Ag specific LAIR-1+/+ CD4 T cells. Although mouse B cells have previously been reported to be negative for LAIR-1 expression, we find that marginal zone B cells express LAIR-1. LAIR-1-/- mice have lower levels of IgG3 and IgG1 in serum and, in response to T-dependent immunization with TNP-OVA, LAIR-1-/- switch less efficiently to IgG2a and IgG2b production, while switching to IgG1 is not affected. Several mouse disease models, including those for experimental autoimmune encephalomyelitis (EAE) and experimental colitis, were utilized to examine the effect of LAIR-1 deficiency in vivo, and no difference in the response of LAIR-1-/- and LAIR-1+/+ mice were observed. Taken together, these observations indicate that LAIR-1 plays a role in shaping the immune response that may be compensated for in vivo by other inhibitory receptors(submitted and in revision). The immunomodulatory receptor CD300a is expressed on human B cells. Nave B cells express very low levels of this receptor, while memory B cells and plasma blasts/cells express variable levels of CD300a. Germinal center B cells are negative for CD300a expression. Stimulation of naive B cells via BCR and TLR9, along with T cell help, failed to up-regulate CD300a cell surface expression despite the increased expression of the memory marker CD27 and the down-regulation of CD305. TLR9 stimulation alone significantly increased CD300a expression on memory B cells, whereas IL-4 and TGF-beta1 act as negative regulators of CD300a expression on memory B cells. Co-ligation of BCR and CD300a inhibits Ca2+ mobilization and NFAT transcriptional activity evoked by BCR ligation alone. Suppression of CD300a expression by primary B cells with siRNA resulted in increased BCR-mediated proliferation, thereby confirming the inhibitory capacity of CD300a. Finally, we show that of CD300a expression levels are down-regulated in the circulating B cells of HIV-infected patients. Altogether, these data demonstrate a novel mechanism for suppressing the activity of B cells and suggest a potential role for CD300a in the B cell dysfunction observed in HIV induced immunodeficiency. Reportedly, CD300f negatively regulates interactions between dendritic and T cells, and acts as an anti-inflammatory molecule in a multiple sclerosis mouse model. We found that a CD300f/Fc chimeric protein specifically binds to apoptotic/dead splenocytes and to apoptotic cells from starved or irradiated lymphocytic cell lines; an observation extended to insect cells. CD300f also binds PMA/ionomycin-activated splenocytes and Ag-stimulated T cells, an interaction inhibited by Annexin V. By ELISA, co-sedimentation and surface plasmon resonance using phospholipid containing liposomes, we show that CD300f preferentially binds phosphatidylserine and requires a metal ion. Exogenous expression of CD300f in cell lines results in enhanced phagocytosis of apoptotic cells. We conclude that expression of CD300f conveys additional capacity to recognize phosphatidylserine to myeloid cells. The result of this recognition may vary with the overall qualitative and quantitative receptor content, as well as signaling capacity of the expressing effector cell, but enhanced phagocytosis is one measurable outcome. Toso is a member of immunoglobulin gene superfamily that is highly expressed on NK cells. We cofirmed that this receptor binds IgM and does not function as an anti-apoptotic molecule. Nothing is known about the function of this receptor on NK cells. Our results show that crosslinking this receptor on primary NK cells leads to phosphorylation of cytoplasmic tyrosine residues, as well as to the activation of intracellular kinases, indicating that ligation of this receptor is capable of activating NK cells. We have also learned that exposure of NK cells to IL-2 markedly down-regulates the expression of FcmuR. Since activated NK cells secrete IL-2, this down-modulation may occur in order to avoid excess inflammation and thus may have functional consequences.