In this study, we implicate OX40 in the severe ocular inflammation of human patients. Moreover, further activation of OX40 significantly exacerbates the severity of EAU. In addition to expanding activated T cells, OX40 can potentially exert its immunologic impact on memory T cells through the signaling of IL-7R, Bcl-6, and Blimp-1.
OX40 is a key costimulatory molecule that is expressed 24 hours after T-cell activation. It has been shown to enhance effector lymphocyte function and to promote memory T-cell development.5,6,9,10
In the B10.RIII EAU model, we found that activation of OX40 during the IRBP161–180
priming phase or at disease onset markedly augments ocular inflammation. This suggests that OX40 not only boosts the antigen priming process but also amplifies the pathologic T-cell response.
It has been shown that both activated effector T cells and Treg express OX40.25
In contrast to our observations, Weinberg et al.17
recently reported that OX40-activating antibody ameliorates experimental autoimmune encephalopathy by expanding Treg numbers during the antigen-sensitization period before the disease onset. We also observed a potentially unique effect of OX40 in the pathogenesis of uveitis. Activation of OX40 at the time of IRBP161–180
immunization markedly extended the disease course of EAU. These findings suggest that aberrant OX40 signaling in uveitis may augment the effector function and longevity of uveitogenic T cells.
T cells can differentiate to Th1, Th2, and Th17 subsets on the basis of distinctive transcription factor and cytokine expression and function. These unique T-cell subsets undertake special immunologic tasks and responsibilities. Adding to the complexity of our immune system, some T cells are found to coexpress cytokines representative of more than one subset. Although we simplistically conceptualize that one distinctive T-cell subset mediates one particular disease, in reality multiple T-cell lineages are often involved in uveitis and other disease processes. Recently, Caspi et al.18
demonstrated that Th1 and Th17 cells are each capable of inducing EAU, depending on different antigen stimulation conditions. OX40 has been shown to promote Th1 and Th2 differentiation.26,27
We have recently reported12
that OX40 also augments Th17 effector function. In this study, we demonstrated that the activation of OX40 enhances the ocular expression of mRNA for IFN-γ and IL-17 in EAU, which suggests that OX40 promiscuously activates different T-cell subsets during inflammation.
After antigen encounter, some activated T cells become long-lasting memory cells that are responsible for the antigen recall response. Both effector and memory T cells contribute to the chronic and relapsing course of uveitis. Consistent with recent published studies,28,29
we found that OX40 agonistic antibody treatment significantly expands CD4+
lymphocytes in the EAU model. In addition, the stimulation of OX40 increases IL-7Rα expression in this activated T-cell population. IL-7 is a common γ (γc) cytokine that plays an indispensable role in memory T-cell development. IL-7 enhances antiapoptotic gene Bcl-2 expression and inhibits proapoptotic factors BAX and BAD.30,31
In addition, the cellular response to IL-7 is regulated by the expression of IL-7R. IL-7R consists of IL-7Rα and the γc chain subunit. Distinct from other γc chain cytokine receptors that are upregulated in activated effector T cells, IL-7R is primarily expressed by naive and memory lymphocytes, suggesting its critical role in supporting these two T-cell populations. Indeed, studies32,33
have demonstrated the dependence of memory T-cell survival on IL-7 and IL-7R. Our study has shown that OX40 primarily upregulates IL-7Rα in CD4+
T cells, suggesting that these activated lymphocytes become memory T cells or memory precursors.
In addition to unique cytokine milieus, T-cell differentiation requires intrinsic signals from master transcription factors. Bcl-6 and Blimp-1 are reciprocal transcription factors that play key roles in determining lymphocyte destiny.34
They were initially found to regulate B- and T-follicle helper cell differentiation. However, the latest studies22–24
demonstrate that Bcl-6 and Blimp-1 ubiquitously control the development of effector and memory CD4+
T cells. Bcl-6 promotes memory T-cell development, whereas Blimp-1 enhances effector T-cell proliferation and function. In addition, γc cytokines have been shown to induce the expression of Bcl-6 and Blimp-1.35
Here, we have shown that the activation of OX40 results in a reciprocal change of Bcl-6 and Blimp-1 in the eyes of the mice developing EAU, thus further supporting the notion that OX40 promotes memory T-cell development in uveitis.
In summary, this study underscores the role of OX40 in the pathogenesis of uveitis. It also implicates OX40 in the development of uveitogenic memory T cells. Although OX40 could directly upregulate IL-7R and Bcl-6 to facilitate the generation of memory lymphocytes, at this time we cannot exclude the possibility that the increase of IL-7R and Bcl-6 levels is secondary to the expansion of memory T cells that express these molecules. This provides a rationale to further study how OX40 regulates memory T-cell development. Further research in this field is important not only for understanding the molecular mechanism of T-cell regulation by OX40 but also for identifying downstream therapeutic targets of OX40 signaling to treat uveitis.