The normal T cell-dependent B cell response must elicit effective immunity while minimizing collateral damage to the host. Loss of control over this finely tuned system can emerge by the failure of a number of regulatory mechanisms that must balance immune system development and activation in a manner that limits autoreactivity and pathological potential. The goal of this study was to evaluate the potential roles for regulatory mechanisms governed by MHC class I proteins in these processes. We chose to study two models – the severe SLE-like disorder in BXSB.Yaa mice and terminal mature B cell lymphomas in the SJL mice, both of which develop as the result of uncontrolled autoreactive B cell responses driven in a CD4+ T cell-dependent manner. Our approach relied on analyses of survival using experimental cohorts sized to ensure robust discriminative power sufficient to detect even subtle effects. To further maximize the cross-comparability of survival data, environmental variation was kept to a minimum by performing all studies in a consistent husbandry environment.
We first assessed the effects on survival of a deficiency in B2M, a genetic manipulation that obviates expression of the great majority of MHC class I family proteins. The most striking result was that BXSB.Yaa
and SJL mice deficient in B2M developed much more aggressive and lethal forms of the distinct diseases that characterize these strains. These results agree with previous findings in the (NZBxNZW)F1 SLE model disease (38
). In contrast,
studies of B2M-deficient MRL.Faslpr
mice showed a reduction in markers of disease, including hypergammaglobulinemia and ANA (39
). However, it is worth noting that other disease phenotypes of MRL.Faslpr
mice, including vasculitis and dermatitis, were exacerbated in B2M-deficient MRL.Faslpr
), findings consistent with a protective role for B2M in these pathological processes.
Discrete deficiencies in the MHC class Ib family members, FCGRT and CD1D1, did not measurably impact the lifespans of BXSB.Yaa
and SJL mice. Studies of mice deficient in FCGRT support a role for this protein in the exacerbation of humorally-mediated autoimmune processes (7
). Studies of CD1D1-deficient (NZBxNZW)F1 mice and pristane-treated BALB/c mice contrast with our findings in that the severity of SLE-like disease was significantly exacerbated in the mutant mice (44
). The varied phenotypic effects on different disease parameters as well as different autoimmune models are certain to reflect the pleiotropic nature of B2M on its MHC family partner proteins as well as the genetic heterogeneity of mouse models of autoimmunity and, likely, of human autoimmune diseases as well. Given this pleiotropy, the overriding phenotype of protection from more lethal forms of the distinct diseases of BXSB.Yaa
and SJL mice that we report is remarkable.
Several convergent lines of evidence indicate that the accelerations of disease and mortality seen in B2M-deficient mice of these and related genetic backgrounds can be ascribed to a marked weakening of a normally potent CD8/NK suppressor axis. First, the development of severe autoimmune disease in (NZW X BXSB.Yaa
)F1 mice was accelerated in mice depleted of CD8+
T cells by treatment with anti-CD8 mAb (46
). Second, weakening of this axis caused by deficiencies in CD8α, TAP1 and H2K/D in BXSB.Yaa
mice and in CD8α in SJL mice resulted in acceleration of their diseases. Third, BXSB.Yaa
mice deficient in IL15, a cytokine that supports both NK cells and memory CD8+
T cells (24
), exhibited a similar acceleration of disease. Finally, as illustrated most forcefully by studies of BXSB.Yaa
mice deficient in both CD8α and IL15, crippling of this axis resulted not only in the halving of lifespans but also in the precocious development of an autoimmune disease that is very similar to that found in B2M-deficient mice and is phenotypically indistinguishable from the more protracted disease characteristic of wt BXSB.Yaa
The commonalities between the accelerated disease of CD8α/IL15-deficient DKO mice and the more protracted wt BXSB.Yaa
disease are exemplified by the transcriptional signature that they share, including upregulated class switched IgG isotypes and the canonical CD4+
. In addition, the strict requirement for IL21 signaling, previously shown to be critical for the autoimmune disease of wt BXSB.Yaa
), was mirrored by absolute dependence on signaling through the IL21R for the accelerated diseases of DKO and B2M-deficient BXSB.Yaa
mice. Finally, the precocious appearance of ICOShi
T cells, activated B cells, elevated serum levels of IgG and ANAs, and the accumulation of extrafollicular plasmablasts and plasma cells all lead us to conclude that the CD8/NK axis, absent in DKO mice, is highly proficient at retarding the earliest stages of disease. We note, however, that the eventual development of autoimmunity in BXSB.Yaa
mice, as well as lymphomas in SJL mice, indicate that this suppression is eventually overwhelmed, either by exhaustion of the regulatory population(s) or by their failure to cope with progressively expanding numbers of target cells.
Parallels between the CD8 component of the regulatory CD8/NK axis described in our studies and CD8+
previously shown to influence a range of autoimmune disorders are readily apparent (reviewed in (47
)). While the phenotypes of CD8+
vary greatly both in rodent models and humans, certain populations express CD122 (29
). The expanded population of CD122+
T cells detected in spleens of BXSB.Yaa
mice as early as 6 wk of age also expressed markers characteristic of CM CD8+
T cells. This putative CD8+
population, which does not express FOXP3, is remarkably similar to a recently described population of CD122+
that are dependent on expression of the MHC class Ib molecule, QA1, and on IL15 for their development and function (31
). These naturally arising CD8+
are thought to respond through their TCR to QA1-bound QDM-like peptides. They also exert perforin-mediated cytotoxic activity against TFH
), a cell type thatis a key source of IL21 that drives BXSB.Yaa
). Taken together, these findings are most consistent with the active involvement of these CD8+
, driven by IL15 and operating at least partially by perforin-mediated lysis, to eliminate autoreactive IL21-expressing TFH
-likecells that drive the severe SLE-like syndrome of BXSB.Yaa
mice. We note that the survival of BXSB.Yaa
mice deficient in TAP1 or CD8α alone, or doubly deficient in TAP1 and CD8α, were all shortened to nearly the same extent (Fig. S1A
). The failure of TAP1 and CD8α deficiencies to complement suggests that the development and/or function of this CD8+
population is dependent on peptides processed by the TAP pathway.
While the above arguments support the involvement of suppressor CD8+
in controlling the BXSB.Yaa
disease, the highly aggressive autoimmune phenotype of B2M-deficient BXSB.Yaa
mice was fully recapitulated only in mice lacking both CD8α and IL15. This result implies that MHC class I-dependent, IL15-dependent suppressor cells not accommodated within the CD8+
paradigm also contributed to disease suppression. One possible explanation for this observation is based on the fact that MHC class I-restricted T cells that resemble conventional CD8+
T cells (CD8 T cell “wannabes”) develop and function in CD8α deficient mice (50
). The IL15-dependent disease restriction observed in the absence of CD8+
T cells may be mediated by CD8+
T cell wannabes with suppressor activity.
Alternatively, the IL15-dependence of suppression found even in the absence of CD8+
T cells may be attributed to conventional NK cells. NK cells dependent on MHC class I molecules are greatly reduced in IL15-deficient mice and operate through a balance of stimulatory and inhibitory innate receptors (24
). Activated and potentially “stressed” lymphocytes that promote autoimmune disease may induce effector function of NK cells. It is noted that the reconstitution of a B2M-deficient mice with a human B2M TG only partially recapitulates the disease resistance observed with wt BXSB.Yaa
mice (Fig. S1B
), despite the fact that the hB2M TG allows for robust positive selection of CD8+
T cells (52
). Human B2M impairs the recognition of mouse MHC Class I proteins by Ly49 (53
). The inability of hB2M TG mB2M-deficient BXSB.Yaa
mice to fully recapitulate the disease phenotype B2M-competent BXSB.Yaa
mice is consistent with the involvement of Ly49as well as conventional NK cells in suppression of autoimmunity in ways that are not currently understood.
Similar to the SLE-like disease of BXSB.Yaa
mice, the late-onset, mature B cell lymphomas of SJL mice develop as a consequence of MHC class II-dependent interactions between autoreactive CD4+
T cells and B cells. The increased risk for the development of B cell NHL in patients with SLE is suggestive of overlapping mechanisms in the pathogenesis of these disorders (55
). Our findings that a CD8/NK axis normally restricts the development of both autoimmunity and lymphomas provide another unifying link between these disorders and, by extension, the potential of common strategies for intervention.
However, a significant difference between the mechanisms governing disease suppression in BXSB.Yaa and SJL mice is that IL15 retarded the development of BXSB.Yaa autoimmunity but not the development of SJL lymphomas. We interpret these differing results to indicate that the cytokine signals that drive the suppressor CD8/NK axis can vary dependent on the nature of the pathological processes and, quite likely, the genetic compositions of the hosts.
Finally, the demonstrated activity of a potent CD8/NK axis in both models of MHC class II-dependent, CD4+ T cell-driven B cell diseases causes us to reconsider the factors that determine autoimmunity and lymphoma. As best exemplified in our detailed studies of BXSB.Yaa mice but extending to the SJL lymphoma-prone mice, any of a variety of mutations that weaken the CD8/NK axis (e.g., the null alleles of Cd8a, H-2K/D, Tap1, Prf1, and Il15 studied here) result in a clinical readout of accelerated and more profound autoimmune disease or precocious development of lymphomas. Given the potent, suppressive effect of the MHC class I-dependent CD8/NK axis described here, any number of genetic or environmental factors that weaken this suppressor axis may shift the tenuous balance towards disease.