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It is well established that viral, parasitic or bacterial infections can prevent type 1 diabetes (T1D) occurrence in non-obese diabetic (NOD) mice. On the other hand, defects in CD4(+) Regulatory T cell (Treg) numbers and/or function contribute to T1D aetiology in NOD mice and in humans. In this work, we formally tested whether the protective role of the bacterial product lipopolysaccharide (LPS) on diabetes incidence results from enhanced Treg activity. We first report that weekly administration of LPS to young prediabetic NOD mice, presenting or not insulitis at the time of treatment, afforded full protection from diabetes. Taking advantage from the high but incomplete penetrance of diabetes in NOD mice raised in specific pathogen free (SPF) conditions we compared untreated disease-free old animals with gender- and age-matched LPS-treated mice. Histological and flow cytometry analysis indicated that LPS treatment did not prevent islet infiltration or priming of diabetogenic T cells but increased Foxp3(+) and CD103(+) Treg frequency and numbers. By performing adoptive transfer experiments into alymphoid NOD/SCID recipients, we further demonstrated that CD25(+) cells from LPS-treated NOD mice, but not from naturally protected animals, maintained diabetogenic cells at check. Our study suggests that T cell regulation represents a cellular mechanism to explain the 'hygiene hypothesis' and reinforces the notion that immune activity consolidates dominant tolerance., (© 2011 The Authors. Scandinavian Journal of Immunology © 2011 Blackwell Publishing Ltd.)

LPS is a strong stimulator of the innate immune system and inducer of B lymphocyte activation. Two TLRs, TLR4 and RP105 (CD180), have been identified as mediators of LPS signaling in murine B cells, but little is known about genetic factors that are able to control LPS-induced cell activation. We performed a mouse genome-wide screen that aside from identifying a controlling locus mapping in the TLR4 region (logarithm of odds score, 2.77), also revealed that a locus closely linked to the MHC region (logarithm of odds score, 3.4) governed B cell responsiveness to LPS stimulation. Using purified B cells obtained from MHC congenic strains, we demonstrated that the MHC(b) haplotype is accountable for higher cell activation, cell proliferation, and IgM secretion, after LPS stimulation, when compared with the MHC(d) haplotype. Furthermore, B cells from MHC class II(-/-) mice displayed enhanced activation and proliferation in response to LPS. In addition, we showed that the MHC haplotype partially controls expression of RP105 (a LPS receptor molecule), following a pattern that resembles the LPS responsiveness phenotype. Together, our results strongly suggest that murine MHC class II molecules play a role in constraining the B cell response to LPS and that genetic variation at the MHC locus is an important component in controlling B cell responsiveness to LPS stimulation. This work raises the possibility that constraining of B cell responsiveness by MHC class II molecules may represent a functional interaction between adaptive and innate immune systems.

Regulatory CD4 T cells (Treg) control inflammatory reactions to commensal bacteria and opportunist pathogens. Activation of Treg functions during these processes might be mediated by host-derived proinflammatory molecules or directly by bacterial products. We tested the hypothesis that engagement of germline-encoded receptors expressed by Treg participate in the triggering of their function. We report that the subset of CD4 cells known to exert regulatory functions in vivo (CD45RB(low) CD25(+)) selectively express Toll-like receptors (TLR)-4, -5, -7, and -8. Exposure of CD4(+) CD25(+) cells to the TLR-4 ligand lipopolysaccharide (LPS) induces up-regulation of several activation markers and enhances their survival/proliferation. This proliferative response does not require antigen-presenting cells and is augmented by T cell receptor triggering and interleukin 2 stimulation. Most importantly, LPS treatment increases CD4(+) CD25(+) cell suppressor efficiency by 10-fold and reveals suppressive activity in the CD4(+) CD45RB(low) CD25(-) subset that when tested ex-vivo, scores negative. Moreover, LPS-activated Treg efficiently control naive CD4 T cell-dependent wasting disease. These findings provide the first evidence that Treg respond directly to proinflammatory bacterial products, a mechanism that likely contributes to the control of inflammatory responses.

Ligation of surface IgM on B cells responding to lipopolysaccharide (LPS) suppresses terminal differentiation and high-rate Ig secretion with no effect on proliferation. As shown here, different B cell populations show characteristic mean values of ligand concentration required for 50% inhibition, with Gaussian distributions of sensitivity to IgM receptor ligation that reflect cellular heterogeneity of 'al-or-none' inhibitions in single cells. Differential sensitivity of B cell populations to IgM ligation seems to be locally determined by the cellular environment and unrelated to the 'maturity' of the responding cells. Thus, while long-lived peritoneal B cells are 3- to 5-fold more resistant than splenic B cells, there is no difference in sensitivity between short- and long-lived B cells in the spleen. Furthermore, while B cells in bone marrow and spleen differ in sensitivity by two orders of magnitude, B cells differentiated in vitro from bone marrow pre-B cells are as resistant as splenic B cells. Moreover, bone marrow cell culture supernatants restore a high level of sensitivity in such cell populations. Differential sensitivity to IgM receptor ligation is reproduced by multivalent nominal antigen, in cell populations that show identical dose-response inhibition curves to direct activation of protein kinase C by phorbol esters. We conclude that the level of sensitivity to IgM ligation is independent of the life span or maturity of the B cell, but differentially regulated in vivo by putative tissue factors. [ABSTRACT FROM AUTHOR]