Universal expression and dual function of the atypical chemokine receptor D6 on innate-like B cells in mice.

Mouse innate-like B cells are a heterogeneous collection of multifunctional cells that control infection, play housekeeping roles, contribute to adaptive immunity, and suppress inflammation. We show that, among leukocytes, chemokine internalization by the D6 receptor is a unique and universal feature of all known innate-like B-cell populations and, to our knowledge, the most effective unifying marker of these cells. Moreover, we identify novel D6(active) B1-cell subsets, including those we term B1d, which lack CD5 and CD11b but exhibit typical B1-cell properties, including spontaneous ex vivo production of IgM, IL-10, and anti-phosphorylcholine antibody. The unprecedented opportunity to examine D6 on primary cells has allowed us to clarify its ligand specificity and show that, consistent with a scavenging role, D6 internalizes chemokines but cannot induce Ca(2+) fluxes or chemotaxis. Unexpectedly, however, D6 can also suppress the function of CXCR5, a critical chemokine receptor in innate-like B-cell biology. This is associated with a reduction in B1 cells and circulating class-switched anti-phosphorylcholine antibody in D6-deficient mice. Therefore, in the present study, we identify a unifying marker of innate-like B cells, describe novel B1-cell subsets, reveal a dual role for D6, and provide the first evidence of defects in resting D6-deficient mice.

An independent subset of TLR expressing CCR2-dependent macrophages promotes colonic inflammation.

Macrophages (Mphis) in the large intestine are crucial effectors of inflammatory bowel disease, but are also essential for homeostasis. It is unclear if these reflect separate populations of Ms or if resident Ms change during inflammation. In this study, we identify two subsets of colonic Ms in mice, whose proportions differ in healthy and inflamed intestine. Under resting conditions, most F4/80+ Ms are TLR- CCR2- CX3CR1hi and do not produce TNF-alpha in response to stimulation. The lack of TLR expression is stable, affects all TLRs, and is determined both transcriptionally and posttranscriptionally. During experimental colitis, TLR2+ CCR2+ CX3CR1int Ly6Chi Gr-1+, TNF-alpha-producing Ms come to dominate, and some of these are also present in the normal colon. The TLR2+ and TLR2- subsets are phenotypically distinct and have different turnover kinetics in vivo, and these properties are not influenced by the presence of inflammation. There is preferential CCR2-dependent recruitment of the proinflammatory population during colitis, suggesting they are derived from independent myeloid precursors. CCR2 knockout mice show reduced susceptibility to colitis and lack the recruitment of TLR2+ CCR2+ Gr-1+, TNF-alpha-producing Ms. The balance between proinflammatory and resident Ms in the colon is controlled by CCR2-dependent recruitment mechanisms, which could prove useful as targets for therapy in inflammatory bowel disease.

The atypical chemokine receptor D6 contributes to the development of experimental colitis.

Proinflammatory CC chemokines control leukocyte recruitment and function during inflammation by engaging chemokine receptors expressed on circulating leukocytes. The D6 chemokine receptor can bind several of these chemokines, but appears unable to couple to signal transduction pathways or direct cell migration. Instead, D6 has been proposed to act as a chemokine scavenger, removing proinflammatory chemokines to dampen leukocyte responses. In this study, we have examined the role of D6 in the colon using the dextran sodium sulfate-induced model of colitis. We show that D6 is expressed in the resting colon, predominantly by stromal cells and B cells, and is up-regulated during colitis. Unexpectedly, D6-deficient mice showed reduced susceptibility to colitis and had less pronounced clinical symptoms associated with this model. D6 deletion had no impact on the level of proinflammatory CC chemokines released from cultured colon explants, or on the balance of leukocyte subsets recruited to the inflamed colon. However, late in colitis, inflamed D6-deficient colons showed enhanced production of several proinflammatory cytokines, including IFN-gamma and IL-17A, and there was a marked increase in IL-17A-secreting gammadelta T cells in the lamina propria. Moreover, Ab-mediated neutralization of IL-17A worsened the clinical symptoms of colitis at these later stages of the response in D6-deficient, but not wild-type, mice. Thus, D6 can contribute to the development of colitis by regulating IL-17A secretion by gammadelta T cells in the inflamed colon.