Metabolically active antigen presenting cells are required for human T cell proliferation in response to the superantigen streptococcal M protein

M protein from type 5 group A streptococci has been identified as a member of the family of polyclonal T cell activators termed superantigens because it preferentially stimulates T cells bearing specific Vβ elements of the T cell receptor (TCR). In this study the molecular and cellular requirements for presentation of this protein to T cells were investigated. Only accessory cells (AC) expressing class II major histocompatibility complex (MHC) molecules were capable of supporting T cell activation in response to a 22 kDa fragment of M protein (pep M). Despite the need for class II elements, processing of pep M5 by the antigen‐presenting cells (APC) was not required for T cell proliferation induced by pep M5. Fixation of APC by paraformaldehyde (PF) treatment impaired their ability to induce optimal T cell proliferation in response to pep M5 withouth significantly affecting interleukin (IL‐2) production. In contrast, PF‐fixation of cells from the B cell lymphoma line, Raji, did not affect their ability to present pep M5 to human T cells. Addition of rIL‐1 and IL‐6 to PF‐treated APC restored pep M5‐induced blastogenesis. Our data suggest that pep M5 directly associates with HLA class II molecules forming a complex that can induce IL‐2 production but not optimal proliferation by T cells. Additional signals provided by the AC are required to trigger optimal T cell proliferation in response to this superantigen.