Strength of TCR signal determines the costimulatory requirements for Th1 and Th2 CD4+ T cell differentiation.

Differentiation of naive CD4 T cells into cytokine-secreting effector Th1 and Th2 cells is influenced by several factors. We have previously reported that the affinity of antigen for TCR and antigen dose can influence the differentiation of Th1 and Th2 cells. Several in vitro and in vivo models have demonstrated a role for the costimulatory molecules, B7-1 (CD80) and B7-2 (CD86), in the generation of distinct effector T cell responses. To determine whether the strength of TCR signaling controls the involvement of CD28 costimulation in selective CD4 T cell differentiation, naive CD4 T cells bearing a transgenic TCR are primed by a weak or strong TCR signal (signal 1) in the presence or absence of B7 costimulatory molecules (signal 2). In this system, IL-4-producing Th2 cells are generated by priming with a weak but not a strong TCR signal. Th2 cell differentiation is dependent on CD28/B7 interactions in that disruption of CD28/B7 interactions inhibits the priming of Th2 cells and cross-linking CD28 with anti-CD28 antibody augments the priming of Th2 cells. In contrast, however, IL-4-producing Th2 cells cannot be generated by priming with a strong TCR signal even in the presence of strong costimulation or high doses of IL-2. Thus, our results suggest that naive CD4 T cells are receptive to CD28-dependent IL-4 production only if they receive a weak TCR signal.

The generation of SDS-stable HLA DR dimers is independent of efficient peptide binding.

MHC class II molecules can exist in two conformations which can be distinguished on the basis of their stability in SDS. The formation of SDS-stable dimers has been shown to correlate with persistent expression of antigen MHC class II-peptide complexes by murine antigen-presenting cells. HLA DR molecules contain either a Val or a Gly at position 86 of the beta chain, which is located in a conserved and prominent hydrophobic pocket in the peptide binding site. Here we show that Val86-containing DR molecules more frequently select peptides which induce the formation of SDS-stable dimers than Gly86 variants. Using analogues of the influence virus haemagglutinin epitope 307-319 we found that the replacement of the aromatic hydrophobic anchor residue (Tyr)at position 309 by amino acids with an aliphatic hydrophobic side chain resulted in the specific formation of high numbers of SDS-stable Val86-DR but not Gly86-DR dimers. These results indicate that the fit between the first anchor residue and the hydrophobic pocket around Dr beta 86 plays a critical role in the formation of SDS-stable DR dimers. Synthetic analogues of naturally processed DR-associated peptides displayed promiscuity in their capacity to bind to several DR specificities and in their ability to induce the SDS-stable conformation. However, no correlation was observed between binding capacity and the ability to induce the SDS-stable conformation. Since it has been shown that SDS stability can relate to the kinetics of peptide-MHC class II interactions, the definition of the requirements for the formation of SDS-stable HLA class II molecules may be important for the design of effective peptide-based immunomodulation protocols.