Nephritis and death were accelerated in CD19−/−
NZB/W mice relative to wild type NZB/W mice () despite B cell hyporesponsiveness and their “immunodeficient” phenotype () of CD19−/−
). These unexpected findings were due to the virtual absence of B10 cells in CD19−/−
NZB/W mice () as described previously for C57BL/6 CD19−/−
). This was confirmed by the adoptive transfer of splenic CD1dhi
B cells from wild type NZB/W mice into CD19−/−
NZB/W mice, which significantly prolonged their survival and demonstrated an important protective role for regulatory B10 cells in this systemic autoimmune disease. Consistent with these observations, B cell depletion by CD20 mAb treatment eliminated 99% of B10 cells and accelerated disease development in young NZB/W mice as demonstrated in the companion paper to these studies (Haas, K. M., R. Watanabe, T. Matsushita, H. Nakashima, N. Ishiura, H. Okochi, M. Fujimoto, and T. F. Tedder. Protective and pathogenic roles for B cells during systemic autoimmunity in NZB/W F1
mice. Submitted). These studies thereby demonstrate protective roles for B cells in lupus pathogenesis.
CD19 expression had both protective and disease promoting roles in lupus pathogenesis in NZB/W mice. CD19-deficiency significantly delayed the generation of ANA, especially anti-dsDNA Abs, in this lupus-prone mouse strain (). Unexpectedly, however, the manifestation of nephritis was paradoxically accelerated by the loss of CD19, although the difference was rather modest (). This result paralleled enhanced mortality in CD19−/−
NZB/W mice. This discrepancy mirrors the findings of Mohan and colleagues in transgenic mice that overexpress CD19 and expressed the Sle1
lupus susceptibility locus (54
). In this case, CD19 overexpression augmented humoral autoimmunity, but did not accelerate mortality or clinical evidence of renal dysfunction. Consistent with this, B cells from these CD19-transgenic mice are hyper-responsive to transmembrane signals, but have significantly increased B10 cell numbers (21
). Thereby, CD19 expression positively correlates with autoantibody production, but is likely to have opposing roles during autoimmune disease by regulating B10 cell development. That severe glomerulonephritis can occur in the absence of ANA, including anti-DNA Abs, and that autoreactive B cells can exert pathogenic effects independent of Ab secretion has also been demonstrated in other lupus-prone mouse strains (55
). Thus, the severe renal disease observed in CD19−/−
NZB/W mice is likely to result from B cell functions other than autoantibody secretion. These studies demonstrate that this B cell function is attributable in part to the “suppressive” role of B10 cells that normally negatively regulate disease progression.
IL-10 is a pleiotropic cytokine with both immunosuppressive and immunostimulatory properties (53
). The role of IL-10 in lupus pathogenesis is complex, including the effects of high serum IL-10 levels in human SLE patients and lupus-prone mouse strains (59
). For example, serum IL-10 levels positively correlate with SLE disease activity scores and anti-dsDNA autoantibody titers, but negatively correlate with C3 and C4 levels and lymphocyte counts (59
). SLE patients also have significantly more IL-10 secreting mononuclear cells in their peripheral blood than normal controls, and disease severity correlates with increased numbers of circulating IL-10-secreting mononuclear cells (61
). Furthermore, IL-10 production by B cells is higher for SLE patients than normal controls, and Ig production by SLE B cells is largely IL-10 dependent (60
). Thereby, IL-10 can be pathogenic for lupus acceleration, but may also be produced to reduce already existing autoimmune inflammation. Various treatments targeting IL-10 against SLE have also shown contradictory results. For instance, IL-10 deficiency significantly enhances disease severity in MRL/lpr mice with increases in IFN-γ and IgG2a anti-dsDNA autoantibody production, which are suppressed by recombinant IL-10 treatment (66
). In the current study, CD19 deficiency led to lower serum IL-10 levels in NZB/W mice throughout the disease course (). By contrast, continuous anti-IL-10 mAb administration significantly delays disease development in NZB/W mice, which is attributed to increased TNF-α production (67
). These contradictory findings are most likely explained by the fact that multiple cell types are capable of producing IL-10, including B cells. Thereby, the positive and negative regulatory roles of IL-10 are likely to differ depending on the cell source of IL-10, as well as the timing of its production, duration, and levels of IL-10 expression. Thus, B10 cell IL-10 production is but one component of a complex regulatory network that balances protective and pathogenic immune responses.
In addition to B10 cells and Ig secretion, B cells regulate immune responses through multiple mechanisms that have only recently been appreciated (68
). B cells contribute to Ag-presentation, cytokine production, the regulation of lymphoid organogenesis, effector T cell differentiation, and dendritic cell function. It is also noteworthy that B cells have other critical roles in lupus, presumably through their interaction with T cells. For instance, B cell deficiency in MRL/lpr mice results in the complete absence of inflammatory T cell renal infiltration (69
). B cell ablation in MRL/lpr mice using CD79 mAb decreases the relative abundance of CD4 memory T cells, and also reduces T cell infiltration into the kidneys (70
). By contrast, MRL/lpr mice engineered to have B cells expressing surface-bound but not secretory Ig develop nephritis which is characterized by renal T cell infiltration (55
). Thus, B cells play pathogenic roles via cytokine secretion or Ag presentation (71
). Since lupus develops under the complex regulation of different B cell subsets and their functions, the selective targeting of B cell subsets may lead to promising therapies for this and other autoimmune disorders.
While the adoptive transfer of CD1dhiCD5+ B cells into CD19−/− NZB/W mice significantly improved survival, this treatment did not “cure” underlying disease (). Since CD19-positive transferred cells were detected in the spleen of CD19−/− NZB/W mice two weeks after injection, but not in five weeks (data not shown), this may be partly explained by the eventual rejection of CD19-expressing wild type B10 cells in CD19-deficient mice. However, this most likely reflects the complex etiology of the lupus-like diseases, and the involvement of multiple hematopoietic lineages in disease initiation and regulation. As an example of this, splenic T cell IL-10 mRNA levels were significantly reduced during the late stages of disease in CD19−/− NZB/W mice (). The spleen CD4+Foxp3+ Treg cell subset was also significantly reduced in CD19−/− NZB/W mice, while Treg cells expanded during disease progression in wild type NZB/W mice (). Consistent with this, the adoptive transfer of CD1dhiCD5+ B cells from wild type NZB/W mice significantly increased Treg cell numbers in CD19−/− NZB/W mice (). These results indicate that CD19 expression by B cells or the presence or absence of B10 cells also has a significant influence on Treg cell development and/or activation in NZB/W mice that remains to be explored. Thus, effective treatments or a cure for lupus-like disease is likely to require the modulation of not only B cell and B10 cell functions, but also T cell and Treg cell functions that significantly modulate disease.