The relative contribution of T cell receptor-versus CD28-mediated signals in co-stimulation of resting CD4 T cells is thought to influence their functional differentiation towards T helper (Th) 1 versus Th2 subsets. We have used a conventional and a mitogenic CD28-specific monoclonal antibody to assess the effect of polyclonal T cell activation through CD28 alone on CD4 subset differentiation. In vivo, mitogenic but not conventional anti-CD28 induces massive lymphocytosis, the Th2 cytokines interleukin (IL)-4 and IL-10, and Th2-dependent immunoglobulin isotypes, most notably IgE. In vitro, it is shown that mitogenic anti-CD28 primes for IL-4-dependent induction of IL-4 expression much more efficiently than conventional co-stimulation. At the molecular level, we show for the first time that the activation of the "Th2 promoting" transcription factor GATA-3 requires co-stimulation by CD28 and is also induced by mitogenic anti-CD28 alone. We suggest that CD28-dependent induction of GATA-3 in concert with other transcription factors, which are preferentially induced by strong CD28-signals, primes CD4 T cells for IL-4-dependent Th2 differentiaton.

Induction of proliferation in primary resting T cells requires engagement of both the antigen‐specific TCR and the co‐stimulatory receptor CD28. Here we report that CD28 functions as an autonomous mitogenic receptor which is mobilized by TCR signaling through cytoskeletal rearrangement. Shortcutting of TCR‐dependent CD28 recruitment by stimulation with monoclonal antibodies specific for mobilized CD28 results in maximum proliferation and IL‐2 secretion in primary resting T cells without activation of ZAP‐70, a central component of the TCR's signal transduction machinery. Engagement of mobilized CD28 fully activates the c‐Jun N‐terminal kinase cascade and translocation of NF‐κB, two key targets of signal integration in co‐stimulation. We propose a two‐step activation model for co‐stimulation in primary resting T cells in which antigen recognition recruits co‐stimulatory receptors which then autonomously transduce signals promoting T cell proliferation.

The relative contribution of T cell receptor‐versus CD28‐mediated signals in co‐stimulation of resting CD4 T cells is thought to influence their functional differentiation towards T helper (Th) 1 versus Th2 subsets. We have used a conventional and a mitogenic CD28‐specific monoclonal antibody to assess the effect of polyclonal T cell activation through CD28 alone on CD4 subset differentiation. In vivo, mitogenic but not conventional anti‐CD28 induces massive lymphocytosis, the Th2 cytokines interleukin (IL)‐4 and IL‐10, and Th2‐dependent immunoglobulin isotypes, most notably IgE. In vitro, it is shown that mitogenic anti‐CD28 primes for IL‐4‐dependent induction of IL‐4 expression much more efficiently than conventional co‐stimulation. At the molecular level, we show for the first time that the activation of the “Th2 promoting” transcription factor GATA‐3 requires co‐stimulation by CD28 and is also induced by mitogenic anti‐CD28 alone. We suggest that CD28‐dependent induction of GATA‐3 in concert with other transcription factors, which are preferentially induced by strong CD28‐signals, primes CD4 T cells for IL‐4‐dependent Th2 differentiaton.