Peptide Requirement for CTL Activation Reflects the Sensitivity to CD3 Engagement: Correlation with CD8αβ Versus CD8αα Expression

Abstract

In our previous studies, CTL that were sensitive to low concentrations of peptide Ag were found to be far superior to those requiring high concentrations of Ag for reducing viral burden when adoptively transferred into SCID mice. Thus it is important that we understand the mechanisms that control the requirement for peptide Ag with the long-term goal of selectively expanding these exquisitely sensitive cells in vivo. Although TCR affinity is one parameter that can affect the CTL sensitivity for Ag, we investigated whether additional mechanisms may also be involved. In studies using a TCR transgenic mouse model, we successfully generated CTL with identical TCR affinity that possess distinctly different activation requirements. Using both peptide Ag and anti-CD3 Ab to activate the CTL lines of high vs low avidity, we found that the variations in activation threshold are the result of differences in the required number of engaged TCR. Additionally, we have observed that the ratio of CD8αβ to CD8αα is significantly greater in CTL lines that are more sensitive to TCR engagement, which may contribute to the lower activation threshold of these CTL following CD3 engagement. These studies identify a novel mechanism by which the activation requirements of Ag-specific CTL are determined by demonstrating a direct correlation between the sensitivity to TCR engagement, the expression of levels CD8αβ vs αα, and the amount of peptide Ag required to reach the threshold for activation.

In vivoModulation of Avidity in Highly Sensitive CD8+Effector T Cells Following Viral Infection

“…Antigen dose has been established as a potent signal that can drive alteration in avidity (3,16,18,36,37). Thus encounter with APC bearing high pMHC in vivo is one potential trigger for reduced avidity.…”

“…Previous studies from our laboratory have established the ability of individual cells to modulate avidity during the initial encounters with antigen (36). Here we tested the hypothesis that these early effectors could give rise to low avidity cells following stimulation with APC bearing high levels of pMHC, a well-established approach for generating low avidity cells in vitro (16)(17)(18)(36)(37)(38). MLN cells were isolated from memory mice on d1 post-secondary infection with PIV5 and CD25 + CD8 + cells sorted.…”

“…At the individual cell level, several factors have been implicated in influencing functional avidity, including TCR affinity and the absolute level or isoform of CD8 expressed (16,18,61). The contribution of TCR affinity to functional avidity has been studied largely through utilization of tetramer dissociation assays as a correlate of TCR affinity (13,61).…”

A correlation between function and selected measures of T cell avidity in influenza virus‐specific CD8+ T cell responses

“…Superior T cell avidity may be explained by various mechanisms, including increased levels of TCR [24], an increased CD8b/CD8a ratio [25,26], increased levels of accessory molecules on the cell surface [27,28], more efficient localization of signaling and accessory molecules to lipid rafts [25,29], and increased levels and/or localization of the Src tyrosine kinase, Lck, one of the first signal transduction molecules activated following TCR ligation [8]. TCRb expression levels were found to be similar on D b NP 366 -and D b PA 224 -specific cells (Fig.…”

DNA vaccination with T‐cell epitopes encoded within Ab molecules induces high‐avidity anti‐tumor CD8+ T cells

“…Several factors have however been implicated in the regulation of functional avidity, e.g. the cytokines IL-12 and IL-15 [10,11], CD8ab expression [7,12], TCR affinity, the level of costimulatory molecules expressed by APC [10,13] and the maturation state of DC. The challenge is therefore to find a vaccine approach that mimics these conditions.…”