Trans‐stimulation of T Cells: Characterization of Targets and Involvement in Loss of Alloreactivity

Abstract

The rapid loss of alloreactivity within populations of antigen-primed, in vitro propagated T cells cannot be explained by the appearance of suppressor cells nor by the dilution effect of the proliferative antigen-specific T cells alone. The involvement of trans-stimulation in loss of alloreactivity, i.e. the recruitment of non-antigen-specific T cells into proliferation in an antigen-dependent and specific fashion, was assessed. Susceptibility to trans-stimulation was found to correlate directly with state of activation at the outset of assay. Large T cells (low buoyant density) but not small T cells (high buoyant density) are susceptible to trans-stimulation. Moreover, in vitro pre-activation of small T cells by mitogen confers susceptibility to trans-stimulation. Analysis of the alloreactivity in Percoll fractions of antigen-primed lymph node T cells revealed activity in both large- and small-T-cell fractions with some enrichment in the latter. The small T cells, refractive to trans-stimulation, are diluted out of the population within the early weeks of antigen-mediated in vitro propagation, accounting for a rapid loss of considerable alloreactivity. The loss of all detectable alloreactivity within antigen-selected populations suggests that the state of activation conferring sensitivity to trans-stimulation must be maintained, and that neither the antigen nor the culture conditions employed met this requirement.