Abstract
Superantigens are microbial proteins that induce massive activation, proliferation, and cytokine production by CD4+ T cells via specific Vβ elements on the TCR. In this study we examine superantigen enhancement of Ag-specific CD4+ T cell activity for humoral B cell responses to T-dependent Ags BSA and HIV gp120 envelope, type I T-independent Ag LPS, and type II T-independent Ag pneumococcal polysaccharides. Injection of BSA followed by a combination of superantigens staphylococcal enterotoxin A and staphylococcal enterotoxin B (SEB) 7 days later enhanced the anti-BSA Ab response in mice ∼4-fold as compared with mice given BSA alone. The anti-gp120 response was enhanced ∼3-fold by superantigens. The type II T-independent Ag pneumococcal polysaccharide response was enhanced ∼2.3-fold by superantigens, whereas no effect was observed on the response to the type I T-independent Ag LPS. The superantigen effect was completely blocked by the CD4+ T cell inhibitory cytokine IL-10. SEB-stimulated human CD4+ T cells were examined to determine the role of the mitogen-activated protein (MAP) kinase signal transduction pathway in superantigen activation of T cells. Inhibitors of the mitogen pathway of MAP kinase blocked SEB-induced proliferation and IFN-γ production, while an inhibitor of the p38 stress pathway had no effect. Consistent with this, SEB activated extracellular signal-regulated kinase/MAP kinase as well as MAP kinase-interacting kinase, a kinase that phosphorylates eIF4E, which is an important component of the eukaryotic protein synthesis initiation complex. Both kinases were inhibited by IL-10. Thus, superantigens enhance humoral immunity via Ag-specific CD4+ T cells involving the stress-independent pathway of MAP kinase. Superantigens are microbial proteins that are potent activators of CD4+ T cells. As such, superantigens can have profound effects on the immune system, both acute and long term (reviewed in Ref. 1). Acute effects include food poisoning and toxic shock syndrome. Long-term effects include autoimmune diseases and immunodeficiency (1). These effects have generally been considered “bad” and “ugly.” However, if the superantigen effects could be harnessed and exploited, then superantigens can have “good” effects for the host, such as enhancement of desirable Ag-specific immune responses. Upon stimulation by superantigens, naive T cells respond and quickly become anergized and/or deleted (2, 3, 4, 5). In contrast, T cells that are actively undergoing activation by specific Ag at the time of superantigen stimulation do not become anergized (6, 7). This is an important characteristic of superantigens that can potentially be exploited when attempting to enhance specific Ag responses. Superantigens can cause anergy and/or deletion of potentially competing naive T cells bearing the same Vβ element(s) as primed T cells of desired Ag specificity. In other words, primed T cells of a desired Ag specificity would be further and more potently expanded by superantigens, while naive T cells of the same Vβ specificity would become anergized. Thus, there would be less “competition” for cytokines and the desired specific immune response would be amplified. We first tested this hypothesis in a mouse model for melanoma (8). B16F10 melanoma is a tumor derived from C57BL/6 mice that has been found to be poorly immunogenic and highly aggressive. We showed that vaccination of mice with a combination of staphylococcal enterotoxin A (SEA),3 staphylococcal enterotoxin B (SEB), and inactivated B16F10 cells led to significant and specific protection against subsequent challenge with viable B16F10 cells (at least 25-fold greater than a lethal dose) (8). Seventy-five percent of mice surviving >170 days remained tumor free after rechallenge with a lethal dose of B16F10, evidence of the development of strong immunologic memory. Additional studies showed increased numbers of CD4+ and CD8+ T cells, CTL activity, and IFN-γ production. Furthermore, failure to produce protection in either CD4−/− or CD8−/− T cell knockout mice is evidence that both CD4+ and CD8+ T cells probably played an essential role in induction of protective immunity. These results showed that superantigen administration subsequent to vaccination with inactivated tumor cells resulted in protective antitumor immunity. In the present study we address the question of whether CD4+ T cell activation by superantigens extends to the subpopulation that provides helper signals for B cell activation and production of Abs to soluble Ags. If superantigens can significantly enhance B cell production of Abs in an Ag-specific manner, then this would suggest that the Th2 subpopulation of CD4+ T cells can also be amplified in an Ag-specific manner. Superantigens would thus be potent activators of both the cellular and humoral arms of the immune system in an Ag-specific manner. This would suggest that superantigens could function as potent novel adjuvants to cellular and humoral immunity against cancer and infectious diseases.