Two intermediate-avidity cytotoxic T lymphocyte clones with a disparity between functional avidity and MHC tetramer staining

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Abstract
The efficacy of cytotoxic T lymphocytes (CTL) has been shown to be highly dependent upon their functional avidity (the sensitivity of their cellular response to MHC–peptide complexes). To examine this relationship, we employed target cell lysis as a quantitative measure and established a set of four CTL clones that exhibited a range of functional avidities spanning more than three orders of magnitude. Within this set, clones displayed a linear correlation between functional avidity and the TCR down-regulation that occurred in response to increasing antigen density. Staining intensity of MHC–peptide tetramer, however, correlated only with the very highest and very lowest avidity clones; the two intermediate-avidity clones showed an inverse relationship between tetramer staining and functional avidity. Compensation for differences in surface levels of TCR improved the correlation, but failed to fully account for this discrepancy. Comparison of TCR signals generated by stimulation of CTL with substrate-bound soluble MHC–peptide or antigen-presenting cells suggested that internal TCR signaling efficiency accounts for at least a portion of the observed functional avidity and suggests the need for caution in directly relating tetramer staining to avidity.