Positive selection of CD4+ T cells by TCR-specific antibodies requires low valency TCR cross-linking: implications for repertoire selection in the thymus

Abstract

The developmental fate of immature CD4+ 8+ thymocytes is determined by intrathymic signals transduced by surface TCR complexes. In particular, TCR signals are required for immature CD4+ 8+ thymocytes to further differentiate into CD4+ 8- or CD4- 8+ T cells, a process referred to as positive selection. It is generally thought that positive selection results from low affinity TCR interactions with self antigens which engage the relatively few surface TCR complexes that are on immature CD4+ 8+ thymocytes. However, we now demonstrate with TCR-specific antibodies that positive selection of CD4+ T cells requires low valency cross-linking of surface TCR complexes on immature thymocytes. That is, positive selection signals are only generated within a narrow range of TCR cross-linking: cross-linking either too few or too many surface TCR complexes fails to signal positive selection. We interpret these results as indicating that positive selection of CD4+ T cells is not signaled by low affinity TCR interactions per se, but rather can be signaled by any combination of TCR affinity and ligand density that induces low valency TCR cross-linking on immature thymocytes.

Different doses of agonistic ligand drive the maturation of functional CD4 and CD8 T cells from immature precursors

“…There is evidence that "strong" TCR ligands (i. e. agonistic ligands with high affinity/low off rates) trigger thymocyte death when present above a critical threshold, whereas "weak" TCR/pMHC interactions (i. e. antagonistic, partially or weakly agonistic ligands with low affinity/high off rates) allow successful thymocyte maturation [3,4]. This suggestion is consistent with studied using TCR/CD3 antibodies in thymic organ culture where thymocyte survival was determined by the degree of TCR cross-linking [11,12]. Interestingly, "strong" but not "weak" pMHC ligands can trigger the aggregation of TCR molecules in solution [9,10], implicating the ligand-induced formation of higher order TCR complexes in the discrimination between positive and negative selection [10].…”

“…3) did not interfere with the generation of SP thymocytes, demonstrating that -in contrast to studies using antibodies to the TCR/CD3 complex [11,12] -additional cross-linking was not detrimental to thymocyte differentiation in this system. g/ml lentil lectin in the example shown in Fig.…”

“…Activation of immature thymocytes by mitogenic plant lectins. First, CD4 T cells can be generated by agonistic antibodies to the TCR/CD3 complex [12,[21][22][23] as long as TCR cross-linking is avoided [12], whereas the antibody-driven generation of CD8 T cells requires the use of CD3 F(ab)' 2 fragments [24] which can antagonize mature T cell activation [25]. g/ml).…”

Extrathymic signals regulate the onset of T cell repertoire selection

“…Consistent with these findings, mAb specific for components of the TCR/CD3 complex have proven ineffective at restoring selection of CD4 -8 + thymocytes in organ cultures of thymi lacking expression of class I [26], highlighting the fundamentally different requirements and signaling pathways required for commitment to either T cell lineage [27,28]. Our results strongly suggest that thymocytes from 14 day embryos may be induced to undergo positive selection into the CD4 + compartment when their TCR are productively engaged, thereby excluding any intrinsic defect in their capacity for transducing appropriate signals.…”

“…Although the failure of one of the five experiments to induce differentiation remains enigmatic, the high degree of concentration dependence for this effect has been noted by others [26], suggesting that even small deviations from the intended optimum may have obscured the outcome.…”