The nonclassical MHC-like molecules of the CD1 family present lipid and glycolipid Ags. Cardiolipin (CL) is a tetra-acylated phospholipid that is a major component of bacterial cell membranes and mitochondrial membranes and is proposed to act as a danger signal to alert the host to infection or tissue damage. In this issue, Dieudé et al. (p. 4771) found that CL is a natural ligand for mouse CD1d and determined the crystal structure of the CL–CD1d complex. T cells specific for CL were found in the liver, and to a lesser degree in the spleen, and rapidly responded to CL stimulation by proliferating and producing IFN-γ. These cells recognized both eukaryotic and prokaryotic CL and were, somewhat surprisingly, found to express γδ TCRs rather than αβ TCRs. CL was presented to these γδ T cells by CD1d, resulting in their activation, as evidenced by the upregulation of CD25 and secretion of RANTES and IFN-γ. The authors found that hepatic γδ T cells were activated in vivo by CD1d-expressing dendritic cells pulsed with CL, but not phosphatidylcholine, supporting the physiologic relevance of these data. These results enhance our understanding of how the immune system detects CL and suggest that CL-specific γδ T cells may be important in immune surveillance for bacterial infection and/or tissue injury.

Innate-like lymphocytes (ILLs) are nonconventional T cells that acquire effector functions without the need for peripheral activation. Mice bearing the Y145F point mutation in the adaptor protein SLP-76 have an expanded population of CD8+ single-positive thymocytes that have characteristics of ILLs and express the transcription factor Eomesodermin (Eomes). Because little is known about this CD8+ ILL population, which is also found in wild-type (WT) mice, Gordon et al. (p. 4573) used the SLP-76 Y145F model to analyze factors involved in the development of these cells. Generation of bone marrow (BM) chimeras with mixed Y145F and WT BM demonstrated that cell-extrinsic factors directed ILL development. In addition to the ILL population, the Y145F mice had an expanded population of γδ T cells secreting IL-4 under the control of the PLZF transcription factor. These γδ T cells likely produced the cell-extrinsic factors suggested by the BM chimera experiments, as both PLZF and IL-4 signaling were found to be required for CD8+ ILL development. T cell-specific deletion of Eomes demonstrated that this transcription factor, a cell-intrinsic factor, was also essential for CD8+ ILL development in both Y145F and WT mice. These data identify a pathway whereby PLZF induces IL-4 production by γδ T cells, leading to Eomes induction and the development of CD8+ ILLs.

The fate of a T cell following Ag stimulation depends not just on the identity of the Ag but also on its concentration. High Ag doses have been shown to inhibit the differentiation of induced regulatory T cells (iTregs), but the mechanism mediating this effect is not known, leading Molinero et al. (p. 4609) to investigate the development of iTregs under different levels of TCR stimulation. Stimulation of naive CD4+ T cells with high doses of anti-CD3 Abs inhibited iTreg generation by preventing Foxp3 transcription in a T cell-intrinsic manner. This inhibition was not due to cell death or impaired TGF-β or IL-2 signaling and could not be fully explained by Akt or mTOR activity. Although NF-κB activity drives Foxp3 expression during the development of natural Tregs in the thymus, this transcription factor was found to be necessary and sufficient to prevent iTreg differentiation in response to high levels of TCR stimulation. This suppression of iTreg differentiation was mediated by production of the NF-κB–dependent cytokines TNF-α and IFN-γ and led these cells to acquire effector, rather than regulatory, characteristics. Thus, in conditions involving high Ag exposure, NF-κB activation promotes a proinflammatory immune response by inducing cytokines that prevent the production of iTregs.

In the NZB/W mouse model of systemic lupus erythematosus, the presence of renal mononuclear cells is associated with nephritis development. To better understand how these cells contribute to disease, Bethunaickan et al. (p. 4994) characterized the populations of macrophages and dendritic cells in NZB/W kidneys. Three populations that increased in frequency during nephritis were identified by the expression of a panel of surface markers and were designated F4/80hi, F4/80lo, and CD11chi. The F4/80hi cells expressed a CD11bhi/CD11cint/Gr1lo/Ly6Clo/VLA4lo/MHCIIhi/CD43lo/CD62Llo phenotype, differing from that of inflammatory macrophages. Upon nephritis induction, these cells were activated, upregulated cathepsin and matrix metalloproteinase activity, and acquired autophagocytic vacuoles. These characteristics were reversed upon disease remission. Peripheral blood Gr1lo monocytes were identified as the precursors of this F4/80hi population. Extensive gene expression analysis of the F4/80hi subset identified a nephritis-associated profile of pro- and anti-inflammatory genes and genes involved in tissue repair and degradation, all of which were downregulated upon remission. Further understanding of this population of renal mononuclear phagocytes may be useful for the therapeutic targeting of the mononuclear cell infiltration that is associated with poor prognosis in human lupus.

Macrophage migration inhibitory factor (MIF) is a broadly expressed proinflammatory molecule that modulates multiple signaling pathways and can promote leukocyte chemotaxis and adhesion. To address how MIF might influence chemokine function, Fan et al. (p. 4915) analyzed its effects on CCL2-mediated monocyte chemotaxis and the involvement of CD74, which binds MIF, in this activity. In vivo, CCL2-induced leukocyte adhesion and recruitment were impaired in mice lacking either MIF or CD74. These data were supported by in vitro observations of reduced CCL2-induced chemotaxis of MIF- or CD74-deficient macrophages, compared with wild-type cells. CD74−/− cells, however, demonstrated significantly increased random migration. These alterations in migration were observed despite normal adhesion molecule and chemokine receptor expression, suggesting they were due to changes in signal transduction. Indeed, MIF−/− and CD74−/− macrophages had reduced levels of CCL2-induced MAPK phosphorylation, RhoA GTPase activity, and actin polymerization, compared with wild-type cells. Resting MIF−/− macrophages expressed elevated levels of the MAPK regulatory protein MAPK phosphatase-1, as well. These data, including an observation that some MIF-mediated promotion of macrophage recruitment occurred in the absence of CD74, support the importance of MIF in cellular migration and suggest that MIF and CD74 have both common and independent functions in chemokine-induced migration.

Tolerance to enteric microbiota must be maintained to avoid the development of inflammatory bowel disease. Expression of the IL-12/23 common subunit p40 (encoded by the Il12b gene) is induced in macrophages by LPS and other microbial stimuli and promotes the development of intestinal inflammation. Kobayashi et al. (p. 4649) investigated whether the LPS-inducible transcription factor NFIL3 (nuclear factor, IL-3 regulated) could be involved in Il12b regulation in the gut. NFIL3 was upregulated in macrophages by LPS or the presence of commensal bacteria, and Il12b expression was dramatically upregulated, compared with that in wild-type cells, in LPS-activated macrophages deficient in NFIL3 or treated with Nfil3-short hairpin RNA. LPS-induced suppression of Il12b expression in wild-type cells required the NFIL3 C-terminal minimal repression domain but was independent of IL-10 expression. The Il12b promoter was found to contain a putative binding motif for NFIL3, and NFIL3 directly interacted with this promoter region to suppress its activity. Interestingly, during the development of colitis in mice, Nfil3 was not expressed in the colon. Correspondingly, lower levels of NFIL3 expression were observed in lamina propria mononuclear cells from patients with Crohn’s disease or ulcerative colitis, compared with those from healthy controls. NFIL3 is thus a transcriptional inhibitor of Il12b that may be important in intestinal homeostasis.

Overgrowth of the anaerobic bacterium Clostridium difficile causes pseudomembranous colitis in individuals treated with antibiotics. Although the prevalence of C. difficile-associated disease has risen over the past decade, the mechanisms involved in host protection against this pathogen are still unclear. Hasegawa et al. (p. 4872) found that the intracellular pattern recognition receptor Nod1 was important for protective immune responses to C. difficile. Toxin-independent recognition of C. difficile by Nod1 specifically stimulated CXCL1 production and neutrophil migration. Compared with wild-type mice, Nod1−/− mice demonstrated increased susceptibility to lethal C. difficile infection despite similar levels of epithelial damage and intestinal permeability, as well as a similar composition of the intestinal microbiota. Nod1 was instead found to enhance neutrophil recruitment to the intestine and prevent bacterial translocation to other organ sites during C. difficile infection. By regulating neutrophil migration via CXCL1, stimulation of Nod1 may help protect against the dangerous consequences associated with severe C. difficile disease.

Clearance of apoptotic cells (AC) leads to tolerance, both by removing autoantigens and by inducing an immunoregulatory milieu characterized by TGF-β and IL-10 production. Because natural, preimmune IgM facilitates the removal of apoptotic cells, Notley et al. (p. 4967) assessed whether preimmune IgM was required for the previously observed ability of AC to suppress inflammatory arthritis. In a model of Ag-induced arthritis that was confirmed to be Ab independent, treatment with ACs suppressed the development of disease and reduced the number of Th17 cells, compared with untreated mice. However, arthritis development and Th17 cell expansion were not affected by AC treatment in mice lacking secreted IgM (Sμ mice). In wild-type but not Sμ mice, AC administration also induced the expansion of IL-10–producing marginal zone B cells in the spleen and IL-10–producing T cells in the lymph nodes. These observations were linked to impaired splenic clearance of ACs by macrophages in Sμ mice. Preincubation of ACs with IgM prior to injection into Sμ mice restored the autoimmunity-suppressing properties of these cells to levels observed in wild-type recipients and resulted in rapid clearance of ACs from Sμ spleens. In this disease model, natural IgM is therefore necessary for the effective clearance of ACs from the spleen and for their ability to suppress the development of arthritis.

Summaries written by Jennifer Hartt Meyers, Ph.D.