Complement opsonization enhances friend virus infection of B cells and thereby amplifies the virus-specific CD8+ T cell response

Friend retrovirus studies reveal complex interactions between intrinsic, innate and adaptive immunity

Approximately 4.4% of the human genome is comprised of endogenous retroviral sequences, a record of an evolutionary battle between man and retroviruses. Much of what we know about viral immunity comes from studies using mouse models. Experiments using the Friend virus (FV) model have been particularly informative in defining highly complex anti-retroviral mechanisms of the intrinsic, innate and adaptive arms of immunity. FV studies have unraveled fundamental principles about how the immune system controls both acute and chronic viral infections. They led to a more complete understanding of retroviral immunity that begins with cellular sensing, production of type I interferons, and the induction of intrinsic restriction factors. Novel mechanisms have been revealed, which demonstrate that these earliest responses affect not only virus replication, but also subsequent innate and adaptive immunity. This review on FV immunity not only surveys the complex host responses to a retroviral infection from acute infection to chronicity, but also highlights the many feedback mechanisms that regulate and counter-regulate the various arms of the immune system. In addition, the discovery of molecular mechanisms of immunity in this model have led to therapeutic interventions with implications for HIV cure and vaccine development.

Infection of B Cell Follicle-Resident Cells by Friend Retrovirus Occurs during Acute Infection and Is Maintained during Viral Persistence

B cell follicles of the spleen and lymph nodes are immune privileged sites and serve as sanctuaries for infected CD4⁺ cells in HIV infection. It is assumed that CD8⁺ T cell responses promote the establishment of the reservoir, as B cell follicles do not permit CD8⁺ T cell entry. Here we analyzed the infected cell population in the Friend retrovirus (FV) infection and investigated whether FV can similarly infect follicular cells. For analysis of FV-infected cells, we constructed a recombinant FV encoding the bright fluorescent protein mWasabi and performed flow cytometry with cells isolated from spleens, lymph nodes and bone marrow of FV-mWasabi-infected mice. Using t-stochastic neighbor embedding for data exploration, we demonstrate how the target cell population changes during the course of infection. While FV was widely distributed in erythrocytes, myeloid cells, B cells, and CD4⁺ T cells in the acute phase of infection, the bulk viral load in the late phase was carried by macrophages and follicular B and CD4⁺ T cells, suggesting that FV persists in cells that are protected from CD8⁺ T cell killing. Importantly, seeding into follicular cells was equally observed in CD8⁺ T cell-depleted mice and in highly FV-susceptible mice that mount a very weak immune response, demonstrating that infection of follicular cells is not driven by immune pressure. Our data demonstrate that infection of cells in the B cell follicle is a characteristic of the FV infection, making this murine retrovirus an even more valuable model for development of retrovirus immunotherapy approaches.IMPORTANCE Human immunodeficiency virus is notorious for its ability to avoid clearance by therapeutic interventions, which is partly attributed to the establishment of reservoirs in latently infected cells and cells that reside in immunologically privileged B cell follicles. In the work presented here, we show that cells of the B cell follicle are equally infected by a simple mouse gammaretrovirus. Using fluorescently labeled Friend retrovirus, we found that B cells and T cells in the B cell follicle, while not carrying the bulk of the virus load, were indeed infected by Friend virus in the early acute phase of the infection and persisted in the chronic infection. Our results suggest that infection of follicular cells may be a shared property of lymphotropic viruses and propose the FV infection of mice as a useful model to study strategies for follicular reservoir elimination.

Fcγ Receptor Type I (CD64)-Mediated Impairment of the Capacity of Dendritic Cells to Activate Specific CD8 T Cells by IgG-opsonized Friend Virus

Dendritic cells (DCs) express Fcγ receptors (FcγRs) for the binding immune complexes (ICs) consisting of IgG and antigens (Ags). IC–FcγR interactions have been demonstrated to enhance activation and antigen-presenting functions of DCs. Utilizing Friend virus (FV), an oncogenic mouse retrovirus, we investigated the effect of IgG-opsonization of retroviral particles on the infection of DCs and the subsequent presentation of viral antigens by DCs to virus-specific CD8 T cells. We found that opsonization by virus-specific non-neutralizing IgG abrogated DC infection and as a consequence significantly reduced the capacity of DCs to activate virus-specific CD8 T cells. Effects of IgG-opsonization were mediated by the high-affinity FcγR type I, CD64, expressed on DCs. Our results suggest that different opsonization patterns on the retroviral surface modulate infection and antigen-presenting functions of DCs, whereby, in contrast to complement, IgG reduces the capacity of DCs to activate cytotoxic T cell (CTL) responses.

Effects of Friend Virus Infection and Regulatory T Cells on the Antigen Presentation Function of B Cells

The primary role of B cells in immunity is considered the production of pathogen-specific antibodies, but another, less-well-studied, function of B cells is to present foreign antigens to T cells to stimulate their activation and proliferation. Dendritic cells (DCs) are considered the most important antigen-presenting cells (APCs) for CD8⁺ T cells, but DCs lose APC function when infected with Friend virus (FV), a model retrovirus of mice. Interestingly, B cells were better able to stimulate CD8⁺ T cell responses when they were infected with FV. We also found that the activation status of B cells under homeostatic conditions was potently modulated by regulatory T cells. This study illustrates an important link between B cell and T cell responses and illustrates an additional mechanism by which regulatory T cells suppress critical T cell responses during viral infections.

Friend virus (FV) is a naturally occurring mouse retrovirus that infects dividing cells of the hematopoietic lineage, including antigen-presenting cells (APCs). The infection of APCs by viruses often induces their dysfunction, and it has been shown that FV infection reduces the ability of dendritic cells (DCs) to prime critical CD8⁺ T cell responses. Nonetheless, mice mount vigorous CD8⁺ T cell responses, so we investigated whether B cells might serve as alternative APCs during FV infection. Direct ex vivo analysis of B cells from FV-infected mice revealed that infected but not uninfected B cells upregulated expression of the costimulatory molecules CD80, CD86, and CD40, as well as major histocompatibility complex class II (MHC-II) molecules. Furthermore, in vitro studies showed that, compared to uninfected B cells from the same mice, the FV-infected B cells had significantly enhanced APC function, as measured by their capacity to prime CD8⁺ T cell activation and proliferation. Thus, in contrast to DCs, infection of B cells with FV enhanced their APC capacity and ability to stimulate the CD8⁺ T cell responses essential for virus control. FV infections also induce the activation and expansion of regulatory T cells (Tregs), so it was of interest to determine the impact of Tregs on B cell activation. The upregulation of costimulatory molecule expression and APC function of B cells was even more strongly enhanced by in vivo depletion of regulatory T cells than infection. Thus, Tregs exert potent homeostatic suppression of B cell activation that is partially overcome by FV infection.

B Cell Requirement for Robust Regulatory T Cell Responses to Friend Retrovirus Infection

Regulatory T cells (Tregs) are immunosuppressive cells of the immune system that control autoimmune reactivity. Tregs also respond during immune reactions to infectious agents in order to limit immunopathological damage from potent effectors such as CD8⁺ cytolytic T lymphocytes. We have used the Friend virus (FV) model of retroviral infection in mice to investigate how viral infections induce Tregs. During acute FV infection, there is significant activation and expansion of thymus-derived (natural) Tregs that suppress virus-specific CD8⁺ T cell responses. Unlike conventional T cells, the responding Tregs are not virus specific, so the mechanisms that induce their expansion are of great interest. We now show that B cells provide essential signals for Treg expansion during FV infection. Treg responses are greatly diminished in B cell-deficient mice but can be restored by adoptive transfers of B cells at the time of infection. The feeble Treg responses in B cell-deficient mice are associated with enhanced virus-specific CD8⁺ T cell responses and accelerated virus control during the first 2 weeks of infection. In vitro experiments demonstrated that B cells promote Treg activation and proliferation through a glucocorticoid-induced receptor superfamily member 18 (GITR) ligand-dependent mechanism. Thus, B cells play paradoxically opposing roles during FV infection. They provide proliferative signals to immunsosuppressive Tregs, which slows early virus control, and they also produce virus-specific antibodies, which are essential for long-term virus control.

When infectious agents invade a host, numerous immunological mechanisms are deployed to limit their replication, neutralize their spread, and destroy the host cells harboring the infection. Since immune responses also have a strong capacity to damage host cells and tissues, their magnitude, potency, and duration are under regulatory control. Regulatory T cells are an important component of this control, and the mechanisms that induce them to respond and exert immunosuppressive regulation are of great interest. In the current report, we show that B cells, the cells responsible for making pathogen-specific antibodies, are also involved in promoting the expansion of regulatory T cells during a retroviral infection. In vitro studies demonstrated that they do so via stimulation of the Tregs through interactions between cell surface molecules: GITR interactions with its ligand (GITRL) on B cells and GITR on regulatory T cells. These findings point the way toward therapeutics to better treat infections and autoimmune diseases.

Narrowing the Gap: Preserving Repertoire Diversity Despite Clonal Selection during the CD4 T Cell Response

T cell immunity relies on the generation and maintenance of a diverse repertoire of T cell antigen receptors (TCRs). The strength of signaling emanating from the TCR dictates the fate of T cells during development, as well as during the immune response. Whereas development of new T cells in the thymus increases the available TCR repertoire, clonal selection during the immune response narrows TCR diversity through the outgrowth of clonotypes with the fittest TCR. To ensure maintenance of TCR diversity in the antigen-selected repertoire, specific mechanisms can be envisaged that facilitate the participation of T cell clonotypes with less than best fit TCRs. Here, we summarize the evidence for the existence of such mechanisms that can prevent the loss of diversity. A number of T cell-autonomous or extrinsic factors can reverse clonotypic hierarchies set by TCR affinity for given antigen. Although not yet complete, understanding of these factors and their mechanism of action will be critical in interventional attempts to mold the antigen-selected TCR repertoire.

Requirement for Fc effector mechanisms in the APOBEC3/Rfv3-dependent neutralizing antibody response

Antiretroviral neutralizing antibody (NAb) responses are often evaluated in the absence of Fc-dependent immune effectors. In murine Friend retrovirus infection, Apobec3/Rfv3 promotes a potent polyclonal NAb response. Here, we show that the Apobec3/Rfv3-dependent NAb response correlated with virus-specific IgG2 titers and that the in vivo neutralization potency of Apobec3/Rfv3-resistant antisera was dependent on activating Fcγ receptors but not complement. The data strengthen retroviral vaccine strategies aimed at eliciting NAbs that activate specific Fcγ receptors.

Humoral immunity in the Friend retrovirus infection model

Major conceptual roadblocks impede the development of an HIV-1 vaccine that can stimulate a potent neutralizing antibody response. Animal models that support HIV-1 replication and allow for host genetic manipulation would be an ideal platform for testing various immunological hypotheses, but progress on this research front has been slow and disappointing. In contrast, many valuable concepts emerged from more than 50 years of studying the Friend retrovirus model. This was recently exemplified by the identification of an innate restriction gene, Apobec3, that could promote the retrovirus-specific neutralizing antibody response. Here we review both classical and recent data on humoral immunity against Friend retrovirus infection, and highlight the potential of this model for unraveling novel aspects of the retrovirus-specific antibody response that may guide HIV-1 vaccine development efforts.

Distinct roles of CD4+ T cell subpopulations in retroviral immunity: lessons from the Friend virus mouse model

It is well established that CD4⁺ T cells play an important role in immunity to infections with retroviruses such as HIV. However, in recent years CD4⁺ T cells have been subdivided into several distinct populations that are differentially regulated and perform widely varying functions. Thus, it is important to delineate the separate roles of these subsets, which range from direct antiviral activities to potent immunosuppression. In this review, we discuss contributions from the major CD4⁺ T cell subpopulations to retroviral immunity. Fundamental concepts obtained from studies on numerous viral infections are presented along with a more detailed analysis of studies on murine Friend virus. The relevance of these studies to HIV immunology and immunotherapy is reviewed.

B cells and TCR avidity determine distinct functions of CD4+ T cells in retroviral infection

The T cell-dependent B cell response relies on cognate interaction between B cells and CD4(+) Th cells. However, the consequences of this interaction for CD4(+) T cells are not entirely known. B cells generally promote CD4(+) T cell responses to pathogens, albeit to a variable degree. In contrast, CD4(+) T cell responses to self- or tumor Ags are often suppressed by B cells. In this study, we demonstrated that interaction with B cells dramatically inhibited the function of virus-specific CD4(+) T cells in retroviral infection. We have used Friend virus infection of mice as a model for retroviral infection, in which the behavior of virus-specific CD4(+) T cells was monitored according to their TCR avidity. We report that avidity for Ag and interaction with B cells determine distinct aspects of the primary CD4(+) T cell response to Friend virus infection. Virus-specific CD4(+) T cells followed exclusive Th1 and T follicular helper (Tfh) differentiation. High avidity for Ag facilitated expansion during priming and enhanced the capacity for IFN-γ and IL-21 production. In contrast, Tfh differentiation was not affected by avidity for Ag. By reducing or preventing B cell interaction, we found that B cells promoted Tfh differentiation, induced programmed death 1 expression, and inhibited IFN-γ production by virus-specific CD4(+) T cells. Ultimately, B cells protected hosts from CD4(+) T cell-mediated immune pathology, at the detriment of CD4(+) T cell-mediated protective immunity. Our results suggest that B cell presentation of vaccine Ags could be manipulated to direct the appropriate CD4(+) T cell response.