CD4 T Cell Dependent Colitis Exacerbation Following Re-Exposure of Mycobacterium avium ssp. paratuberculosis.

Mycobacterium avium ssp. paratuberculosis (MAP) is the causative agent of Johne's disease (JD), a chronic inflammatory bowel disease of cattle characterized by intermittent to chronic diarrhea. In addition, MAP has been isolated from Crohn's disease (CD) patients. The impact of MAP on severity of clinical symptoms in JD as well as its role in CD are yet unknown. We have previously shown that MAP is able to colonize inflamed enteric tissue and to exacerbate the inflammatory tissue response (Suwandi et al., 2014). In the present study, we analyzed how repeated MAP administration influences the course of dextran sulfate sodium (DSS)-induced colitis. In comparison to mice exposed to DSS or MAP only, repeated exposure of DSS-treated mice to MAP (DSS/MAP) revealed a significantly enhanced clinical score, reduction of colon length as well as severe CD4+ T cell infiltration into the colonic lamina propria. Functional analysis identified a critical role of CD4+ T cells in the MAP-induced disease exacerbation. Additionally, altered immune responses were observed when closely related mycobacteria species such as M. avium ssp. avium and M. avium ssp. hominissuis were administered. These data reveal the specific ability of MAP to aggravate intestinal inflammation and clinical symptoms. Overall, this phenotype is compatible with similar disease promoting capabilites of MAP in JD and CD.

Experimental colitis is exacerbated by concomitant infection with Mycobacterium avium ssp. paratuberculosis.

BACKGROUND: Crohn's disease (CD) is a chronic inflammatory disorder of the human gastrointestinal tract. Although genetic, immunological, environmental, and bacterial factors have been implicated, the pathogenesis is incompletely understood. The histopathological appearance of CD strikingly resembles Johne's disease, a ruminant inflammatory bowel disease, caused by Mycobacterium avium ssp. paratuberculosis (MAP), but a causative role of MAP in CD has not been established. In this work, we hypothesized that MAP might exacerbate an already existing intestinal disease. METHODS: We combined dextran sulfate sodium (DSS)-induced colitis with MAP infection in mice and monitored the immune response and bacterial count in different organs. RESULTS: An increased size of liver and spleen was observed in DSS-treated and MAP-infected animals (DSS + MAP) as compared with DSS-treated uninfected (DSS + PBS) mice. Similarly, DSS treatment increased the number and size of MAP-induced liver granulomas and enhanced the MAP counts in enteric tissue. MAP infection in turn delayed the mucosal healing of DSS-induced tissue damage. Finally, high numbers of MAP were found in mesenteric fat tissue causing large granuloma and necrotic regions. CONCLUSIONS: Taken together, we present an in vivo model to study the role of MAP infection in CD. Our results confirm the hypothesis that MAP is able to exacerbate existing intestinal inflammation.

An intrinsic propensity of murine peritoneal B1b cells to switch to IgA in presence of TGF-ß and retinoic acid.

AIMS: In the present study we have investigated the comparative switching propensity of murine peritoneal and splenic B cell subpopulations to IgA in presence of retinoic acid (RA) and TGF-ß. METHODS AND RESULTS: To study the influence of RA and TGF-ß on switching of B cell subpopulations to IgA, peritoneal (B1a, B1b and B2 cells) and splenic (B1a, marginal zone, and B2) B cells from normal BALB/c mice were FACS purified, cultured for 4 days in presence of RA and TGF-ß and the number of IgA producing cells was determined by ELISPOT assay or FACS analysis. In presence of TGF-ß, peritoneal B1b cells switched to IgA more potently than other peritoneal B cell subpopulations. When TGF-ß was combined with retinoic acid (RA), switching to IgA was even more pronounced. Under these conditions, "innate" B cells like peritoneal and splenic B1 cells and MZ B cells produced IgA more readily than B2 cells. Additionally, high frequency of nucleotide exchanges indicating somatic hypermutation in VH regions was observed. Besides IgA induction, RA treatment of sorted PEC and splenic B cells led to expression of gut homing molecules - α4ß7 and CCR9. Intraperitoneal transfer of RA-treated B1 cells into Rag1(-/-) recipients resulted in IgA in serum and gut lavage, most efficiently amongst B1b cell recipients. CONCLUSION: Present study demonstrates the differential and synergistic effect of RA and TGF-ß on switching of different B cell subpopulations to IgA and establishes the prominence of peritoneal B1b cells in switching to IgA under the influence of these two factors. Our study extends our knowledge about the existing differences among B cell subpopulations with regards to IgA production and indicates towards their differential contribution to gut associated humoral immunity.

B-1-cell subpopulations contribute differently to gut immunity.

In mice, B-1 (B1a/B1b) cells are mainly located in the peritoneal cavity. B-1 cells are well known for their role in the early stages of Ab-mediated immune responses against pathogenic invasion as well as for the production of natural IgM antibodies. Although such B cells have been claimed to give rise to intestinal plasma cells producing IgA, a clear role of B-1 cells in IgA production in the gut-associated tissues is still not defined. Here, we employed the transgenic L2 mouse model characterized by the lack of B-2 cells and presence of B-1 cells as major B-cell subpopulation. The oligoclonality of the Ab repertoire in this mouse allowed us to take typical B1a cell VH sequences as indicators of the presence of IgM-producing B-1a cells in Peyer's patches as well as in lamina propria. However, amongst the IgAVH sequences recovered from the same tissues, none of the sequences showed B1a-cell specificity. Interestingly, all IgAVH sequences derived from the lamina propria of L2 mice displayed extensive numbers of nucleotide exchanges, indicating somatic hypermutation, and affinity maturation. This suggests that the contribution of natural unmutated IgA by B-1a cells to intestinal immunity is negligible.

Severe Developmental B Lymphopoietic Defects in Foxp3-Deficient Mice are Refractory to Adoptive Regulatory T Cell Therapy.

The role of Foxp3-expressing regulatory T (T(reg)) cells in tolerance and autoimmunity is well-established. However, although of considerable clinical interest, the role of T(reg) cells in the regulation of hematopoietic homeostasis remains poorly understood. Thus, we analysed B and T lymphopoiesis in the scurfy (Sf) mouse model of T(reg) cell deficiency. In these experiments, the near-complete block of B lymphopoiesis in the BM of adolescent Sf mice was attributed to autoimmune T cells. We could exclude a constitutive lympho-hematopoietic defect or a B cell-intrinsic function of Foxp3. Efficient B cell development in the BM early in ontogeny and pronounced extramedullary B lymphopoietic activity resulted in a peripheral pool of mature B cells in adolescent Sf mice. However, marginal zone B and B-1a cells were absent throughout ontogeny. Developmental B lymphopoietic defects largely correlated with defective thymopoiesis. Importantly, neonatal adoptive T(reg) cell therapy suppressed exacerbated production of inflammatory cytokines and restored thymopoiesis but was ineffective in recovering defective B lymphopoiesis, probably due to a failure to compensate production of stroma cell-derived IL-7 and CXCL12. Our observations on autoimmune-mediated incapacitation of the BM environment in Foxp3-deficient mice will have direct implications for the rational design of BM transplantation protocols for patients with severe genetic deficiencies in functional Foxp3(+) T(reg) cells.

Induction of B-cell development in adult mice reveals the ability of bone marrow to produce B-1a cells.

To study B-cell development from bone marrow (BM), we generated recombination-activating gene 1 (Rag1)-targeted mice lacking mature lymphocytes. B-cell development can be induced in such mice by B cell-specific restoration of a functional Rag1 transcription unit. Follicular and marginal zone B cells populated the spleen when Rag1 expression was permitted. Notably, the peritoneal cavity was dominated by bona fide B-1a cells, as judged by surface markers and functional properties. These BM-derived B-1a cells exhibited a polyclonal VDJ repertoire with substantial N nucleotide insertions. Nevertheless, physiologic frequencies of phosphatidylcholine-specific B cells were detected. Importantly, the BM of young and 5-month-old mice was indistinguishable with regard to the potential to generate B-1a cells.

Somatic hypermutation in peritoneal B1b cells.

Murine B1 cells have been shown to be able to switch to IgA in vitro. In agreement, we could demonstrate in the peritoneum of mice the presence of IgA producing B1 cells. Interestingly, enzyme-linked immunospot assays of lipopolysaccharide stimulated cultures revealed that only the B1b cell subpopulation contained high numbers of such cells while IgA producing B cells were rare amongst the B2 and B1a cell populations. This was confirmed by RT-PCR on sorted peritoneal B cell subpopulations. In addition, the variable regions associated with IgA of peritoneal B1b cells displayed extensive variation due to somatic hypermutation. In contrast, mutations were found only at low frequencies in VH regions associated with IgM of both B1 cell populations. Thus, peritoneal B1b cells display many similarities to B2 cells. This finding is consistent with the idea of a layered immune system in which peritoneal B1a and splenic follicular B2 cells appear at the two extremes and peritoneal B1b and B2 cells represent intermediates.