Regulatory CD8+ T cells associated with erosion of immune surveillance in persistent virus infection suppress in vitro and have a reversible proliferative defect

Control of B Cell Lymphoma by Gammaherpesvirus-Induced Memory CD8 T Cells

Persistent infection with gammaherpesviruses (γHV) can cause lymphomagenesis in immunocompromised patients. Murine γHV-68 (MHV-68) is an important tool for understanding immune factors contributing to γHV control; however, modeling control of γHV-associated lymphomagenesis has been challenging. Current model systems require very long incubation times or severe immune suppression, and tumor penetrance is low. In this report, we describe the generation of a B cell lymphoma on the C57BL/6 background, which is driven by the Myc oncogene and expresses an immunodominant CD8 T cell epitope from MHV-68. We determined MHV-68-specific CD8 T cells in latently infected mice use either IFN-γ or perforin/granzyme to control γHV-associated lymphoma, but perforin/granzyme is a more potent effector mechanism for lymphoma control than IFN-γ. Consistent with previous reports, CD4-depleted mice lost control of virus replication in persistently infected mice. However, control of lymphoma remained intact in the absence of CD4 T cells. Collectively, these data show the mechanisms of T cell control of B cell lymphoma in γHV-infected mice overlap with those necessary for control of virus replication, but there are also important differences. This study establishes a tool for further dissecting immune surveillance against, and optimizing adoptive T cell therapies for, γHV-associated lymphomas.

Murine γ-Herpesvirus 68 Induces Severe Lung Inflammation in IL-27-Deficient Mice with Liver Dysfunction Preventable by Oral Neomycin

IL-27 is an immunoregulatory cytokine consisting of p28 and EBI3. Its receptor also has two subunits, WSX1 and gp130. Although IL-27 promotes Th1 differentiation in naive T cells, it also induces IL-10 expression in effector Th1 cells to curtail excessive immune responses. By using p28-deficient mice and WSX1-deficient mice (collectively called IL-27-deficient mice), we examined the role of IL-27 in primary infection by murine γ-herpesvirus 68 (MHV68), a murine model of EBV. Upon airway infection with MHV68, IL-27-deficient mice had more aggravated lung inflammation than wild-type mice, although MHV68 infection per se was better controlled in IL-27-deficient mice. Although epithelial cells and alveolar macrophages were primarily infected by MHV68, interstitial macrophages and dendritic cells were the major producers of IL-27. The lung inflammation of IL-27-deficient mice was characterized by more IFN-γ-producing CD8⁺ T cells and fewer IL-10-producing CD8⁺ T cells than that of wild-type mice. An infectious mononucleosis-like disease was also aggravated in IL-27-deficient mice, with prominent splenomegaly and severe hepatitis. Infiltration of IFN-γ-producing effector cells and upregulation of the CXCR3 ligand chemokines CXCL9, CXCL10, and CXCL11 were noted in the liver of MHV68-infected mice. Oral neomycin effectively ameliorated hepatitis, with decreased production of these chemokines in the liver, suggesting that the intestinal microbiota plays a role in liver inflammation through upregulation of these chemokines. Collectively, IL-27 is essential for the generation of IL-10-producing effector cells in primary infection by MHV68. Our findings may also provide new insight into the mechanism of hepatitis associated with infectious mononucleosis.

Cytomegalovirus infection in HIV-infected and uninfected individuals is characterized by circulating regulatory T cells of unconstrained antigenic specificity

Cytomegalovirus (CMV) infection is associated with immune-suppression in immune-compromised hosts and old adults. We previously showed that ex vivo CMV restimulation of peripheral blood mononuclear cells (PBMC) of CMV-seropositive volunteers expanded CD4+CD27-CD28- regulatory T cells (Tregs). Here we evaluate the phenotype and function of circulating CD4+CD27-CD28- T cells of CMV-seropositive adults. Compared with CMV-seronegative, CMV-seropositive adults had 10-fold higher CD4+CD27-CD28-% T cells in PBMC. Circulating CD4+CD27-CD28- T cells from both CMV-seropositive and seronegative donors expressed higher levels of TGFβ, granzyme B, CD39, CD147 and IL-35, and lower levels of CD127, compared with their parent circulating CD4+ T cells. However, only CMV-seropositive circulating CD4+CD27-CD28- had increased FOXP3 expression. CD4+CD27-CD28- sorted from the PBMC of CMV-seropositive donors expanded ex vivo in the presence of rhIL2 and inhibited ex vivo proliferation of autologous PBMC restimulated with CMV, varicella-zoster virus or C. albicans antigens. CD4+CD27-CD28- sorted from CMV-seronegative PBMC did not expand in the presence of rhIL2 and did not inhibit autologous PBMC proliferation. CD3+CD27-CD28- circulating T cells (≥80% CD8+) from CMV-seropositive HIV-infected donors also inhibited ex vivo proliferation of autologous PBMC restimulated with CMV or HIV. These data indicate that CMV-seropositive individuals have circulating Tregs that inhibit cell-mediated immune responses to CMV and other antigens and may be contribute to an immune-suppressive effect of CMV infection. Moreover, the phenotypic similarity between circulating CD4+CD27-CD28- Tregs with differentiated effector T cells suggests that the two T-cell subsets might evolve in parallel or in sequence from the same progenitor cells in response to CMV stimulation during reactivations.

Ablation of STAT3 in the B Cell Compartment Restricts Gammaherpesvirus Latency In Vivo

A challenging property of gammaherpesviruses is their ability to establish lifelong persistence. The establishment of latency in B cells is thought to involve active virus engagement of host signaling pathways. Pathogenic effects of these viruses during latency or following reactivation can be devastating to the host. Many cancers, including those associated with members of the gammaherpesvirus family, Kaposi’s sarcoma-associated herpesvirus and Epstein-Barr virus, express elevated levels of active host signal transducer and activator of transcription-3 (STAT3). STAT3 is activated by tyrosine phosphorylation in response to many cytokines and can orchestrate effector responses that include proliferation, inflammation, metastasis, and developmental programming. However, the contribution of STAT3 to gammaherpesvirus pathogenesis remains to be completely understood. This is the first study to have identified STAT3 as a critical host determinant of the ability of gammaherpesvirus to establish long-term latency in an animal model of disease. Following an acute infection, murine gammaherpesvirus 68 (MHV68) established latency in resident B cells, but establishment of latency was dramatically reduced in animals with a B cell-specific STAT3 deletion. The lack of STAT3 in B cells did not impair germinal center responses for immunoglobulin (Ig) class switching in the spleen and did not reduce either total or virus-specific IgG titers. Although ablation of STAT3 in B cells did not have a global effect on these assays of B cell function, it had long-term consequences for the viral load of the host, since virus latency was reduced at 6 to 8 weeks postinfection. Our findings establish host STAT3 as a mediator of gammaherpesvirus persistence.

The insidious ability of gammaherpesviruses to establish latent infections can have detrimental consequences for the host. Identification of host factors that promote viral latency is essential for understanding latency mechanisms and for therapeutic interventions. We provide the first evidence that STAT3 expression is needed for murine gammaherpesvirus 68 to establish latency in primary B cells during an active immune response to infection. STAT3 deletion in B cells does not impair adaptive immune control of the virus, but loss of STAT3 in B cells has a long-lasting impact on viral persistence. These results indicate a potential therapeutic benefit of STAT3 inhibitors for combating gammaherpesvirus latency and, thereby, associated pathologies.

Functional heterogeneity in the CD4+ T cell response to murine γ-herpesvirus 68

CD4(+) T cells are critical for the control of virus infections, T cell memory, and immune surveillance. We studied the differentiation and function of murine γ-herpesvirus 68 (MHV-68)-specific CD4(+) T cells using gp150-specific TCR-transgenic mice. This allowed a more detailed study of the characteristics of the CD4(+) T cell response than did previously available approaches for this virus. Most gp150-specific CD4(+) T cells expressed T-bet and produced IFN-γ, indicating that MHV-68 infection triggered differentiation of CD4(+) T cells largely into the Th1 subset, whereas some became follicular Th cells and Foxp3(+) regulatory T cells. These CD4(+) T cells were protective against MHV-68 infection in the absence of CD8(+) T cells and B cells, and protection depended on IFN-γ secretion. Marked heterogeneity was observed in the CD4(+) T cells, based on lymphocyte Ag 6C (Ly6C) expression. Ly6C expression positively correlated with IFN-γ, TNF-α, and granzyme B production; T-bet and KLRG1 expression; proliferation; and CD4(+) T cell-mediated cytotoxicity. Ly6C expression inversely correlated with survival, CCR7 expression, and secondary expansion potential. Ly6C(+) and Ly6C(-) gp150-specific CD4(+) T cells were able to interconvert in a bidirectional manner upon secondary Ag exposure in vivo. These results indicate that Ly6C expression is closely associated with antiviral activity in effector CD4(+) T cells but is inversely correlated with memory potential. Interconversion between Ly6C(+) and Ly6C(-) cells may maintain a balance between the two Ag-specific CD4(+) T cell populations during MHV-68 infection. These findings have significant implications for Ly6C as a surface marker to distinguish functionally distinct CD4(+) T cells during persistent virus infection.

Immune escape of γ-herpesviruses from adaptive immunity

Epstein-Barr virus (EBV) and Kaposi's sarcoma-associated herpesvirus (KSHV) are two γ-herpesviruses identified in humans and are strongly associated with the development of malignancies. Murine γ-herpesvirus (MHV-68) is a naturally occurring rodent pathogen, representing a unique experimental model for dissecting γ-herpesvirus infection and the immune response. These γ-herpesviruses actively antagonize the innate and adaptive antiviral responses, thereby efficiently establishing latent or persistent infections and even promoting development of malignancies. In this review, we summarize immune evasion strategies of γ-herpesviruses. These include suppression of MHC-I-restricted and MHC-II-restricted antigen presentation, impairment of dendritic cell functions, downregulation of costimulatory molecules, activation of virus-specific regulatory T cells, and induction of inhibitory cytokines. There is a focus on how both γ-herpesvirus-derived and host-derived immunomodulators interfere with adaptive antiviral immunity. Understanding immune-evasive mechanisms is essential for developing future immunotherapies against EBV-driven and KSHV-driven tumors.