These analyses were carried out using cells from a TCR transgenic model and as such, the divergence in peptide sensitivity among cells was not the result of differences in TCR affinity. Given a constant TCR affinity, the molecular basis for the significant difference in peptide requirement between high and low avidity cells generated in this model is intriguing. In the present, study we used a high and low avidity cell line generated from OT-I TCR transgenic mice to probe TCR signalling following avidity tuning. In addition to sharing a common TCR, the two lines used here bind similar amounts

of tetramer. Although not directly demonstrated, these results are consistent with a similar capacity to engage pMHC. Alectinib price At the initiation of these studies we proposed two general hypotheses that could account for the increased peptide requirement by low avidity cells: (i) low avidity cells require a greater magnitude of TCR-generated signal to activate effector functions, i.e. cytokine production and lytic granule release, or (ii) high and low avidity cells

require a similar level of signalling to elicit effector Ulixertinib function, but a greater amount of pMHC is required to achieve this threshold. Here we show that the requirement for increased peptide in low avidity cells is not the result of a difference in the quantity of downstream signal necessary for activation (as measured by erk phosphorylation and intracellular calcium levels). In fact, we also observed similar patterns of activation in the upstream molecules in the pathway, i.e. LAT and CD3ζ, in high and low avidity cells under threshold conditions for effector function. The requirement for similar levels of signalling appears to generally be the case, as comparable findings for erk activation were obtained in two independently generated pairs of lines (data not shown). Instead, our results are consistent with a requirement for increased TCR engagement to achieve initiation of the requisite level of signalling. This model is supported by the enough finding that the low avidity cells require a greater amount of anti-CD3 to promote IFN-γ production compared with the high avidity cells. We have previously reported that

high and low avidity lines generated in the TCR transgenic model exhibit differences in CD8 expression at the cell surface.10,12 Changes in CD8 can manifest as differences in the absolute level of CD8, with lower avidity cells exhibiting reduced levels of both CD8α and -β or, more often in our hands, in the relative expression of CD8α versus -β, with low avidity cells having a lower β : α ratio.11,12 A decreased β : α ratio in low avidity cells is consistent with a greater proportion of CD8 expressed as αα homodimers. The high and low avidity lines used in this study represent the latter regulation, exhibiting differences in CD8β expression in the face of similar CD8α levels, thereby resulting in a reduced β : α ratio in the low avidity cells.