OX2 (CD200) is a broadly expressed membrane glycoprotein, shown here to be important for regulation of the macrophage lineage. In mice lacking CD200, macrophage lineage cells, including brain microglia, exhibited an activated phenotype and were more numerous. Upon facial nerve transection, damaged CD200-deficient neurons elicited an accelerated microglial response. Lack of CD200 resulted in a more rapid onset of experimental autoimmune encephalomyelitis (EAE). Outside the brain, disruption of CD200-CD200 receptor interaction precipitated susceptibility to collagen-induced arthritis (CIA) in mice normally resistant to this disease. Thus, in diverse tissues OX2 delivers an inhibitory signal for the macrophage lineage.

The OX2 membrane glycoprotein (CD200) is expressed on a broad range of tissues including lymphoid cells, neurons, and endothelium. We report the characterization of an OX2 receptor (OX2R) that is a novel protein restricted to cells of the myeloid lineage. OX2 and its receptor are both cell surface glycoproteins containing two immunoglobulin-like domains and interact with a dissociation constant of 2.5 microM and koff 0.8 s(-1), typical of many leukocyte protein membrane interactions. Pervanandate treatment of macrophages showed that OX2R could be phosphorylated on tyrosine residues. Blockade of the OX2-OX2R interaction with an OX2R mAb exacerbated the disease model experimental allergic encephalomyelitis. These data, together with data from an OX2-deficient mouse (R. M. Hoek et al., submitted), suggest that myeloid function can be controlled in a tissue-specific manner by the OX2-OX2R interaction.

Specialized roles for the pro-inflammatory cytokines tumor necrosis factor (TNF) and lymphotoxin (LT) were characterized in TNF/LT alpha -/- and TNF -/- mice established by direct gene targeting of C57BL/6 ES cells. The requirement for LT early in lymphoid tissue organogenesis is shown to be distinct from the more subtle and varied role of TNF in promoting correct microarchitectural organization of leukocytes in LN and spleen. Development of normal Peyer's patch (PP) structure, in contrast, is substantially dependent on TNF. Only mice lacking LT exhibit retarded B cell maturation in vivo and serum immunoglobulin deficiencies. A temporal hierarchy in lymphoid tissue development can now be defined, with LT being an essential participant in general lymphoid tissue organogenesis, developmentally preceeding TNF that has a more varied and subtle role in promotion of correct spatial organization of leukocytes in LN and spleen PP development in TNF -/- mice is unusual, indicating that TNF is a more critical participant for this structure than it is for other lymphoid tissues.

Mice without secreted TNF but with functional, normally regulated and expressed membrane-bound TNF (memTNF(Delta/Delta) mice) were created by knocking-in the uncleavable Delta 1-9,K11E TNF allele. In contrast to TNF-deficient mice (TNF(-/-)), memTNF supported many features of lymphoid organ structure, except generation of primary B cell follicles. Splenic chemokine expression was near normal. MemTNF-induced apoptosis was mediated through both TNF-R1 and TNF-R2. That memTNF is suboptimal for development of inflammation was revealed in experimental autoimmune encephalomyelitis. Disease severity was reduced in memTNF(Delta/Delta) mice relative to wild-type mice, and the nature of spinal cord infiltrates resembled that in TNF(-/-) mice. We conclude that memTNF supports many processes underlying lymphoid tissue structure, but secreted TNF is needed for optimal inflammatory lesion development.

DAP12 is an ITAM-bearing membrane adaptor molecule implicated in the activation of NK and myeloid cells. In mice rendered DAP12 deficient by targeted gene disruption, lymphoid and myeloid development was apparently normal, although the activating Ly49 receptors on NK cells were downregulated and nonfunctional. To analyze the consequences of DAP12 deficiency in vivo, we examined the susceptibility of DAP12-/- mice to experimental autoimmune encephalomyelitis (EAE). DAP12-/- mice were resistant to EAE induced by immunization with myelin oligodendrocyte glycoprotein (MOG) peptide. Resistance was associated with a strongly diminished production of IFNgamma by myelin-reactive CD4+ T cells due to inadequate T cell priming in vivo. These data suggest that DAP12 signaling may be required for optimal antigen-presenting cell (APC) function or inflammation.