In priming cultures of CD4 T cells from TCR transgenic mice, low concentrations of agonist peptide strikingly favorsTh2 differentiation. Similarly, priming with high concentrations of partial agonists (altered peptide ligands) leads to a Th2 biased response, suggesting that low receptor occupancy during priming predisposes to Th2 differentiation, independent of exogenous IL-4(1
). The suppression of Th2 responses by TCR stimulations resulting from high receptor occupancy appears in part to be due to increased ERK phosphorylation that in turn suppressesGATA-3 transcription(3
. Indeed, at high peptide concentration, T cells from TCR transgenic mice generally differentiate into Th1 cells. Other evidence for an association between weak TCR signaling and Th2 differentiation can be found in multiple studies showing that hypomorphic mutations in TCR signaling moleculescan lead to severe atopic or Th2-linked disease(4
). Furthermore, the Th2-promoting mechanism of the schistosome egg antigen component Omega-1 appears to be attenuation of the antigen-presenting potency of dendritic cells(8
In polyclonal responses, it is anticipated that there will exist CD4+ cells with a hierarchy of affinities to a given peptide-MHC complex. It might be anticipated that the lower affinity members of the population would, as the result of lower receptor occupancy during priming, differentiate to Th2 cells while the higher affinity members would acquire a Th1 phenotype. However, what is generally observed is that in such populations, T-cells of high affinity respond robustly while cells whose TCRs provide interactions with the immunizing peptidethat is below certain affinity/occupancy thresholds either react minimally or not at all in these mixed populations(9
). The failure of these “lower affinity” cells to respond could be due to competition from higher affinity members of the population or due to cell intrinsic properties. In the absence of high affinity cells, it would be expected that naive low affinity cells, if they can respond at all, would acquire a Th2 phenotype since they would achieve low receptor occupancy evenat high pPCC concentrations.
To determine whether such low affinity cells could respond in the absence of high affinity cells and how they would differentiate, we utilized CD4T cells from 5CC7 Vβ3 transgenic mice, which have normal endogenous, polyclonal TCRα chain rearrangement and a fixed β chain derived from a TCR specific for the PCC peptide 89–104 (pPCC). The T-cells from these donors contain a polyclonal population of which ~5% have been estimated to be specific for pPCC. The higher affinity members of this population were depleted by cell sorting after binding of a pMCC I-Ek tetramer. When the remaining cells, containing both low affinity cells and cells that were unreactive with pPCC, were primed with high concentrations of pPCC, substantial IL-4 production was observed. Furthermore, priming single cells from Vα11Vβ3 TCR transgenic donors, which are of uniform high affinity for pPCC, with high peptide concentrations resulted in very few clones that made substantialIL-4. By contrast, when single cells derived from polyclonal 5CC7 Vβ3 donors were primed with high concentrations of peptide, the majority of the emerging clones made significant amounts of IL-4. TCRα-chain sequence analysis showed that those clones that possessed TCR motifs known to be associated with high affinity responses to PCC made significantly less IL-4 then clones that possessed fewer motifs associated with high affinity. Finally, naive T-cells from Vβ3 transgenic mice expressing a PCC transgene that thymically eliminates most PCC-reactive T-cells were more likely to make IL-4 when primed with high peptide concentrations than were cells derived from Vβ3 TCR transgenics that lacked the PCC transgene. Collectively, there results indicate that among polyclonal cells, those that bear TCRs with low affinity for peptide preferentially adopt a Th2 phenotype when they are primed in the absence of competition from cells of the same specificity that have high affinity for ligand. Even small numbers of naive T-cells with high affinity for a given antigen inhibit Th2 differentiation by the low affinity cells within the cultured population or prevent the outgrowth of these cells.