| Though the importance of both the TCR and CD28 signaling molecules to T cell clonal expansion are well known, defining the precise nature of this regulation still requires more work. We utilized both in vitro and in vivo models of CD4+ T cell activation to study cell cycle progression. We found that, in vitro, only when either IL-2R signaling or a continuous presence of TCR and CD28 ligands were present were CD4+ T cells capable of maintaining a maximal rate of cell division, dependent on the PI3K/mTOR signaling pathway. In vivo however, unlike in vitro, an increase in the antigen dose not only regulated the percentage of responders as seen in vitro, but also the quality of that response. This decrease in proliferative capacity upon low antigen dose administration resulted in only minimally slowed rates of initiation of both TCR signaling and cell cycle progression. The rate of cell cycle progression, however, upon stimulation at saturating antigen doses was over three times greater than under limiting antigen doses. This data correlated with the loss of pMHC availability under limiting antigen concentrations in vivo. Therefore, the rate of cell cycle progression seemed to be regulated at both early and late timepoints by the density of pMHC available. To further test the role of TCR signaling at later timepoints during primary CD4+ T cell activation, we next utilized two different models to inhibit TCR signaling after an initial stimulation period. First, upon in vitro pre-activation of CD4+ T cells and subsequent transfer in vivo, CD4+ T cells were only able to continue to proliferate at a maximal rate when cognate pMHC ligands were present. Also, utilizing a mAB specific for the pMHC ligand of a TCR-tg CD4+ T cell, we showed that CD4+ T cells are continually sensitive to the presence of pMHC ligands. Upon loss of pMHC availability, CD4+ T cells rapidly lost cell size and slowed their proliferative response. Therefore, CD4+ T cells are continually sensitive to the presence of pMHC ligands and adjust their cell size and rate of cell cycle progression according to the density of pMHC available. |
