Functional characterization of IgA-targeted bacterial taxa from undernourished Malawian children that produce diet-dependent enteropathy.

To gain insights into the interrelationships among childhood undernutrition, the gut microbiota, and gut mucosal immune/barrier function, we purified bacterial strains targeted by immunoglobulin A (IgA) from the fecal microbiota of two cohorts of Malawian infants and children. IgA responses to several bacterial taxa, including Enterobacteriaceae, correlated with anthropometric measurements of nutritional status in longitudinal studies. The relationship between IgA responses and growth was further explained by enteropathogen burden. Gnotobiotic mouse recipients of an IgA(+) bacterial consortium purified from the gut microbiota of undernourished children exhibited a diet-dependent enteropathy characterized by rapid disruption of the small intestinal and colonic epithelial barrier, weight loss, and sepsis that could be prevented by administering two IgA-targeted bacterial species from a healthy microbiota. Dissection of a culture collection of 11 IgA-targeted strains from an undernourished donor, sufficient to transmit these phenotypes, disclosed that Enterobacteriaceae interacted with other consortium members to produce enteropathy. These findings indicate that bacterial targets of IgA responses have etiologic, diagnostic, and therapeutic implications for childhood undernutrition.

The neuropeptide neuromedin U promotes autoantibody-mediated arthritis.

INTRODUCTION: Neuromedin U (NMU) is a neuropeptide with pro-inflammatory activity. The primary goal of this study was to determine if NMU promotes autoantibody-induced arthritis. Additional studies addressed the cellular source of NMU and sought to define the NMU receptor responsible for its pro-inflammatory effects. METHODS: Serum containing arthritogenic autoantibodies from K/BxN mice was used to induce arthritis in mice genetically lacking NMU. Parallel experiments examined whether NMU deficiency impacted the early mast-cell-dependent vascular leak response induced by these autoantibodies. Bone-marrow chimeric mice were generated to determine whether pro-inflammatory NMU is derived from hematopoietic cells or stromal cells. Mice lacking the known NMU receptors singly and in combination were used to determine susceptibility to serum-transferred arthritis and in vitro cellular responses to NMU. RESULTS: NMU-deficient mice developed less severe arthritis than control mice. Vascular leak was not affected by NMU deficiency. NMU expression by bone-marrow-derived cells mediated the pro-arthritogenic effect. Deficiency of all of the known NMU receptors, however, had no impact on arthritis severity and did not affect the ability of NMU to stimulate intracellular calcium flux. CONCLUSIONS: NMU-deficient mice are protected from developing autoantibody-induced inflammatory arthritis. NMU derived from hematopoietic cells, not neurons, promotes the development of autoantibody-induced inflammatory arthritis. This effect is mediated by a receptor other than the currently known NMU receptors.

Peripheral education of the immune system by colonic commensal microbiota.

The instruction of the immune system to be tolerant of self, thereby preventing autoimmunity, is facilitated by the education of T cells in a specialized organ, the thymus, in which self-reactive cells are either eliminated or differentiated into tolerogenic Foxp3(+) regulatory T (T(reg)) cells. However, it is unknown whether T cells are also educated to be tolerant of foreign antigens, such as those from commensal bacteria, to prevent immunopathology such as inflammatory bowel disease. Here we show that encounter with commensal microbiota results in the peripheral generation of T(reg) cells rather than pathogenic effectors. We observed that colonic T(reg) cells used T-cell antigen receptors (TCRs) different from those used by T(reg) cells in other locations, implying an important role for local antigens in shaping the colonic T(reg)-cell population. Many of the local antigens seemed to be derived from commensal bacteria, on the basis of the in vitro reactivity of common colon T(reg) TCRs. These TCRs did not facilitate thymic T(reg)-cell development, implying that many colonic T(reg) cells arise instead by means of antigen-driven peripheral T(reg)-cell development. Further analysis of two of these TCRs by the creation of retroviral bone marrow chimaeras and a TCR transgenic line revealed that microbiota indigenous to our mouse colony was required for the generation of colonic T(reg) cells from otherwise naive T cells. If T cells expressing these TCRs fail to undergo T(reg)-cell development and instead become effector cells, they have the potential to induce colitis, as evidenced by adoptive transfer studies. These results suggest that the efficient peripheral generation of antigen-specific populations of T(reg) cells in response to an individual's microbiota provides important post-thymic education of the immune system to foreign antigens, thereby providing tolerance to commensal microbiota.