Aberrant wound healing and TGF-beta production in the autoimmune-prone MRL/+ mouse.

Wound healing is a complex process that involves inflammation, apoptosis, growth, and tissue remodeling. The autoimmune-prone inbred mouse strain MRL/+ manifests accelerated and extensive healing to ear punch wounds, suggesting a link between immune defects and wound healing. Prior studies with lupus-prone mice have shown that hematopoietic cells of lupus-prone strains can transfer disease to otherwise non-autoimmune-prone recipients. In this study we performed reciprocal bone marrow transfers between MRL and the control strain B10.BR and found that radioresistant MRL/+ host cells, rather than hematopoietic cells, are required for the healing response. We have also made the novel observations that, compared to normal controls, MRL/+ hematopoietic cells overproduce TGF-beta1 and manifest impaired inflammatory responses to lipopolysaccharide challenge. These features suggest that the aberrant wound healing phenotype of MRL mice is independent of their propensity to develop autoimmunity.

Efficient peripheral clonal elimination of B lymphocytes in MRL/lpr mice bearing autoantibody transgenes.

Peripheral B cell tolerance was studied in mice of the autoimmune-prone, Fas-deficient MRL/ lpr.H-2(d) genetic background by introducing a transgene that directs expression of membrane-bound H-2Kb antigen to liver and kidney (MT-Kb) and a second transgene encoding antibody reactive with this antigen (3-83mu delta, anti-Kk,b). Control immunoglobulin transgenic (Ig-Tg) MRL/lpr.H-2(d) mice lacking the Kb antigen had large numbers of splenic and lymph node B cells bearing the transgene-encoded specificity, whereas B cells of the double transgenic (Dbl-Tg) MRL/lpr.H-2(d) mice were deleted as efficiently as in Dbl-Tg mice of a nonautoimmune B10.D2 genetic background. In spite of the severely restricted peripheral B cell repertoire of the Ig-Tg MRL/lpr.H-2(d) mice, and notwithstanding deletion of the autospecific B cell population in the Dbl-Tg MRL/lpr.H-2(d) mice, both types of mice developed lymphoproliferation and exhibited elevated levels of IgG anti-chromatin autoantibodies. Interestingly, Dbl-Tg MRL/lpr.H-2(d) mice had a shorter lifespan than Ig-Tg MRL/lpr.H-2(d) mice, apparently as an indirect result of their relative B cell lymphopenia. These data suggest that in MRL/lpr mice peripheral B cell tolerance is not globally defective, but that certain B cells with receptors specific for nuclear antigens are regulated differently than are cells reactive to membrane autoantigens.

A functional B cell receptor transgene allows efficient IL-7-independent maturation of B cell precursors.

IL-7 supports the proliferation of B cell precursors, but inhibits their maturation to mature surface IgM+ (sIgM+) B cells. This inhibition is thought to occur by direct or indirect down-regulation of recombinase genes, preventing the B cells from undergoing Ig light chain rearrangements. To directly analyze the IL-7 inhibitory effects, we studied B cell development and maturation in B cells bearing a transgenic (Tg) B cell receptor (BCR). We show here that proliferation of Tg B cell precursors is IL-7 dependent both in vivo and in vitro and is comparable to that of non-Tg B cell precursors. Tg B cell precursors grown on stroma and IL-7 expressed sIgM on >90% of the cells, and a large proportion of these cells coexpressed additional maturation markers such as IgD, CD23, CD21, and L-selectin, indicating that IL-7 does not inhibit maturation of Tg B cell precursors. The presence of the Tg inhibited V(D)J recombination in the cultured cells, as very low levels of recombination activating genes 2 (RAG-2) expression and endogenous V-Jkappa DNA rearrangements were found. Expression levels of RAG mRNAs were not significantly changed after removal of IL-7 from the in vitro Tg B cell cultures. In contrast, we found that IL-7 inhibited maturation of non-Tg B cell precursors and that removal of IL-7 resulted in a significant increase in RAG-2 expression and kappa rearrangements, thus allowing the B cells to express sIgM and to mature. These results suggest that IL-7-mediated inhibition of Ig gene rearrangement blocks maturation of B cell precursors and that the presence of Tg BCR efficiently circumvents this inhibition.