Th2 activities induced during virgin T cell priming in the absence of IL-4, IL-13, and B cells

Virgin T cells being primed to Th2-inducing or Th1-inducing Ags, respectively, start to synthesize IL-4 or IFN-gamma as they begin to proliferate. Parallel respective induction of B cells to produce gamma1 or gamma2a switch transcripts provides additional evidence of early divergent Th activity. This report concerns the roles of IL-4, IL-13, and B cells in these early events in vivo. Th2 responses were induced in lymph nodes against hapten-protein given s.c. with killed Bordetella pertussis adjuvant. In T cell proliferation in wild-type mice, IL-4 message up-regulation and gamma1 and epsilon switch transcript production were underway 48-72 h after immunization. The absence of IL-4, IL-13, or B cells did not alter the early T cell proliferative response. The gamma1 and epsilon switch transcript production was still induced in the absence of IL-4, IL-13, or both, but at a reduced level, while the dominance of switching to IgG1 in the extrafollicular hapten-specific plasma cell response was retained. The up-regulation of IL-4 message was not reduced or delayed in the absence of B cells and was only marginally reduced by the absence of IL-13. It is concluded that signals delivered by dendritic cells, which are not dependent on the presence of IL-4, IL-13, or B cells, can prime virgin T cells and induce the early Th2 activities studied. These early events that direct virgin T cells toward Th2 differentiation contrast with the critical later role of Th2 cytokines in selectively expanding Th2 clones and driving further IL-4 synthesis.

Preferential infection of immature dendritic cells and B cells by mouse mammary tumor virus

Until now it was thought that the retrovirus mouse mammary tumor virus preferentially infects B cells, which thereafter proliferate and differentiate due to superantigen-mediated T cell help. We describe in this study that dendritic cells are infectable at levels comparable to B cells in the first days after virus injection. Moreover, IgM knockout mice have chronically deleted superantigen-reactive T cells after MMTV injection, indicating that superantigen presentation by dendritic cells is sufficient for T cell deletion. In both subsets initially only few cells were infected, but there was an exponential increase in numbers of infected B cells due to superantigen-mediated T cell help, explaining that at the peak of the response infection is almost exclusively found in B cells. The level of infection in vivo was below 1 in 1000 dendritic cells or B cells. Infection levels in freshly isolated dendritic cells from spleen, Langerhans cells from skin, or bone marrow-derived dendritic cells were compared in an in vitro infection assay. Immature dendritic cells such as Langerhans cells or bone marrow-derived dendritic cells were infected 10- to 30-fold more efficiently than mature splenic dendritic cells. Bone marrow-derived dendritic cells carrying an endogenous mouse mammary tumor virus superantigen were highly efficient at inducing a superantigen response in vivo. These results highlight the importance of professional APC and efficient T cell priming for the establishment of a persistent infection by mouse mammary tumor virus.

Role of dendritic cells in the immune response induced by mouse mammary tumor virus superantigen

After mouse mammary tumor virus (MMTV) infection, B lymphocytes present a superantigen (Sag) and receive help from the unlimited number of CD4(+) T cells expressing Sag-specific T-cell receptor Vbeta elements. The infected B cells divide and differentiate, similarly to what occurs in classical B-cell responses. The amplification of Sag-reactive T cells can be considered a primary immune response. Since B cells are usually not efficient in the activation of naive T cells, we addressed the question of whether professional antigen-presenting cells such as dendritic cells (DCs) are responsible for T-cell priming. We show here, using MMTV(SIM), a viral isolate which requires major histocompatibility complex class II I-E expression to induce a strong Sag response in vivo, that transgenic mice expressing I-E exclusively on DCs (I-EalphaDC tg) reveal a strong Sag response. This Sag response was dependent on the presence of B cells, as indicated by the absence of stimulation in I-EalphaDC tg mice lacking B cells (I-EalphaDC tg muMT(-/-)), even if these B cells lack I-E expression. Furthermore, the involvement of either residual transgene expression by B cells or transfer of I-E from DCs to B cells was excluded by the use of mixed bone marrow chimeras. Our results indicate that after priming by DCs in the context of I-E, the MMTV(SIM) Sag can be recognized on the surface of B cells in the context of I-A. The most likely physiological relevance of the lowering of the antigen threshold required for T-cell/B-cell collaboration after DC priming is to allow B cells with a low affinity for antigen to receive T-cell help in a primary immune response.

Cutting Edge: Apoptosis of Superantigen-Activated T Cells Occurs Preferentially After a Discrete Number of Cell Divisions In Vivo

Staphylococcal enterotoxins are bacterial products that display superantigen activity in vitro as well as in vivo. For instance, staphylococcal enterotoxin B (SEB) polyclonally activates T cells that bear the Vβ8 gene segment of the TCR. SEB-activated T cells undergo a burst of proliferation that is followed by apoptosis. Using an in vivo adaptation of a fluorescent cell division monitoring technique, we show here that SEB-activated T cells divide asynchronously, and that apoptosis of superantigen-activated T cells is preferentially restricted to cells which have undergone a discrete number of cell divisions. Collectively, our data suggest that superantigen-activated T cells are programmed to undergo a fixed number of cell divisions before undergoing apoptosis. A delayed death program may provide a mechanistic compromise between effector functions and homeostasis of activated T cells.

Cutting edge: apoptosis of superantigen-activated T cells occurs preferentially after a discrete number of cell divisions in vivo

Staphylococcal enterotoxins are bacterial products that display superantigen activity in vitro as well as in vivo. For instance, staphylococcal enterotoxin B (SEB) polyclonally activates T cells that bear the Vbeta8 gene segment of the TCR. SEB-activated T cells undergo a burst of proliferation that is followed by apoptosis. Using an in vivo adaptation of a fluorescent cell division monitoring technique, we show here that SEB-activated T cells divide asynchronously, and that apoptosis of superantigen-activated T cells is preferentially restricted to cells which have undergone a discrete number of cell divisions. Collectively, our data suggest that superantigen-activated T cells are programmed to undergo a fixed number of cell divisions before undergoing apoptosis. A delayed death program may provide a mechanistic compromise between effector functions and homeostasis of activated T cells.

Interplays between mouse mammary tumor virus and the cellular and humoral immune response

Mouse mammary tumor virus has developed strategies to exploit the immune response. It requires vigorous immune stimulation to achieve efficient infection. The infected antigen-presenting cells present a viral superantigen on the cell surface which stimulates strong CD4-mediated T-cell help but CD8 T-cell responses are undetectable. Despite the high frequency of superantigen-reactive T cells, the superantigen-induced immune response is comparable to classical antigen responses in terms of T-cell priming, T-cell-B-cell collaboration as well as follicular and extra-follicular B-cell differentiation. Induction of systemic anergy is observed, similar to classical antigen responses where antigen is administered systemically but does not influence the role of the superantigen-reactive T cells in the maintenance of the chronic germinal center reaction. So far we have been unable to detect a cytotoxic T-cell response to mouse mammary tumor virus peptide antigens or to the superantigen. This might yet represent another step in the viral infection strategy.

A highly sensitive in vitro infection assay to explore early stages of mouse mammary tumor virus infection

Mouse mammary tumor virus (MMTV) infection of adult mice induces a strong response to superantigen (Sag) in their draining lymph nodes, which results from the presentation of Sag by MMTV-infected B cells to Sag-reactive T cells. To date, infection with physiologically relevant doses of MMTV can be detected in vivo only after several days of Sag-mediated T-cell-dependent amplification of infected B cells. Furthermore, no efficient in vitro system of detecting MMTV infection is available. Such a model would allow the dissection of the early phase of infection, the assessment of the contributions of different cell types, and the screening of large panels of molecules for their potential roles in infection and Sag response. For these reasons, we have established an in vitro model for detecting infection which is as sensitive and reproducible as the in vivo model. We found that the viral envelope (Env) protein is crucial for target cell infection but not for presentation of Sag. Furthermore, we show that infection of purified B cells with MMTV induces entry of Sag-responsive T cells into the cell cycle, while other professional antigen-presenting cells, such as dendritic cells, are much less efficient in inducing a response.