Induction of yeast mating pheromone a-factor by alpha cells

Saccharomyces cerevisiae cells of a and alpha mating types constitutively secrete cell-specific peptide mating pheromones. a-Factor is secreted by a cells and acts on alpha cells, while alpha-factor is secreted by alpha cells and acts on a cells. Confirming preliminary studies, we demonstrate here that cultures of a cells contain higher than constitutive levels of a-factor activity when grown with alpha cells or alpha-factor. This induction of a-factor may result from increased synthesis or increased secretion of a-factor, as opposed to modification or stabilization of preexisting a-factor, as part of the a cell response to alpha-factor, as an a ste2 mutant (which cannot respond to alpha-factor) is not induced by alpha-factor. In mixed cultures inoculated with equal numbers of a cells and alpha cells, a cells predominate by stationary phase. Thus, a series of sequential interactions between a and alpha cells may be involved in establishing optimal hormone concentrations and cell ratios for conjugation.     

Qa-1 restricted recognition of foreign antigen by a gamma delta T-cell hybridoma

Distinct T-lymphocyte subsets recognize antigens in conjunction with different classes of major histocompatibility complex (MHC) glycoproteins using the T-cell receptor (TCR), a disulphide-linked heterodimer associated with the CD3 complex on the cell surface. In general, class I and class II MHC products provide a context for the recognition of foreign antigens by CD8+ and CD4+ T cells, respectively. This recognition seems to be largely dependent on alpha beta TCR heterodimers, whereas the function of the second gamma delta TCR, present on a minor subpopulation of cells, is still unknown. In the mouse, the existence of six cell-surface MHC class I products (K, D, L, Qa-1, Qa-2 and Tla) has been firmly established by serological, biochemical and genetic evidence. So far, only the most polymorphic of them, K, D and L ('classical' class I) have been reported as restriction elements for T-cell recognition of foreign antigens. The function of the relatively invariant Qa and Tla molecules remains unknown. We have made a T-helper cell hybridoma clone (DGT3) that recognizes synthetic copolymer poly(Glu50Tyr50) in the context of Qa-1 cell surface product, and has a CD4-CD8- phenotype. Our studies indicate that DGT3 cells express the gamma delta TCR on the cell surface, implicating its role in Qa-1-restricted antigen recognition. This is the first evidence that T cells can recognize foreign antigen in association with self Qa product, confirming that Qa molecules not only topologically, but also functionally, belong to the MHC.