A unique unresponsive CD4+ T cell phenotype post TCR antagonism

Imprinting and Editing of the Human CD4 T Cell Response to Influenza Virus

Immunity to influenza is unique among pathogens, in that immune memory is established both via intermittent lung localized infections with highly variable influenza virus strains and by intramuscular vaccinations with inactivated protein-based vaccines. Studies in the past decades have suggested that the B cell responses to influenza infection and vaccination are highly biased by an individual's early history of influenza infection. This reactivity likely reflects both the competitive advantage that memory B cells have in an immune response and the relatively limited diversity of epitopes in influenza hemagglutinin that are recognized by B cells. In contrast, CD4 T cells recognize a wide array of epitopes, with specificities that are heavily influenced by the diversity of influenza antigens available, and a multiplicity of functions that are determined by both priming events and subsequent confrontations with antigens. Here, we consider the events that prime and remodel the influenza-specific CD4 T cell response in humans that have highly diverse immune histories and how the CD4 repertoire may be edited in terms of functional potential and viral epitope specificity. We discuss the consequences that imprinting and remodeling may have on the potential of different human hosts to rapidly respond with protective cellular immunity to infection. Finally, these issues are discussed in the context of future avenues of investigation and vaccine strategies.

Dual TCR T Cells: Identity Crisis or Multitaskers?

Dual TCR T cells are a common and natural product of TCR gene rearrangement and thymocyte development. As much as one third of the T cell population may have the capability to express two different TCR specificities on the cell surface. This discovery provoked a reconsideration of the classic model of thymic selection. Many potential roles for dual TCR T cells have since been hypothesized, including posing an autoimmune hazard, dominating alloreactive T cell responses, inducing allergy, and expanding the TCR repertoire to improve protective immunity. Yet, since the initial wave of publications following the discovery of dual TCR T cells, research in the area has slowed. In this study, we aim to provide a brief but comprehensive history of dual TCR T cell research, re-evaluate past observations in the context of current knowledge of the immune system, and identify key issues for future study.

Subtle changes in TCRα CDR1 profoundly increase the sensitivity of CD4 T cells

Changes in the peptide and MHC molecules have been extensively examined for how they alter T cell activation, but many fewer studies have examined the TCR. Structural studies of how TCR differences alter T cell specificity have focused on broad variation in the CDR3 loops. However, changes in the CDR1 and 2 loops can also alter TCR recognition of pMHC. In this study we focus on two mutations in the CDR1α loop of the TCR that increased the affinity of a TCR for agonist Hb(64-76)/I-E(k) by increasing the on-rate of the reaction. These same mutations also conferred broader recognition of altered peptide ligands. TCR transgenic mice expressing the CDR1α mutations had altered thymic selection, as most of the T cells were negatively selected compared to T cells expressing the wildtype TCR. The few T cells that escaped negative selection and were found in the periphery were rendered anergic, thereby avoiding autoimmunity. T cells with the CDR1α mutations were completely deleted in the presence of Hb(64-76) as an endogenous peptide. Interestingly, the wildtype T cells were not eliminated, identifying a threshold affinity for negative selection where a 3-fold increase in affinity is the difference between incomplete and complete deletion. Overall, these studies highlight how small changes in the TCR can increase the affinity of TCR:pMHC but with the consequences of skewing selection and producing an unresponsive T cell.

T cell recognition of weak ligands: roles of signaling, receptor number, and affinity

T cell recognition of antigen is a crucial aspect of the adaptive immune response. One of the most common means of pathogen immune evasion is mutation of T cell epitopes. T cell recognition of such ligands can result in a variety of outcomes including activation, apoptosis and anergy. The ability of a given T cell to respond to a specific peptide-MHC ligand is regulated by a number of factors, including the affinity, on- and off-rates and half-life of the TCR-peptide-MHC interaction. Interaction of T cells with low-potency ligands results in unique signaling patterns and requires engagement with a larger number of T cell receptors than agonist ligands. This review will address these aspects of T cell interaction with weak ligands and the ways in which these ligands have been utilized therapeutically.