Thoughts on Positive Selection in Thymus

An observation that illustrates most T cell receptor structure-function relationships

The Standard model of the T cell receptor (TCR) structure-function relationships is based on an analogy with the B cell receptor. Here a single observation is analyzed to show why this appears to be untenable. The Standard model cannot account for allele-specific recognition of theMHC-encoded presenter of peptide (R) by the TCR nor can it adequately explain alloreactivity. The competing framework is based on the assumptions that (1) single V-domains recognize the alleles of R, (2) restrictive reactivity is peptide specific, whereas alloreactivity is peptide unspecific, and (3) the TCR is born in two conformations, which display reciprocal behaviors (see text). In any case, whatever position one takes regarding these two models, competing conceptualizations are of crucial value in guiding experimentation, not to mention creative thinking.

Signaling interactions predicted by the Tritope model of the TCR

As the data accumulates, it becomes obvious that the Standard Model of TCR structure-function relationships is in jeopardy. The proposed Tritope model has become more meaningful and, in any case, is richer in prediction and explanation. This is illustrated here by using the signaling interactions of the TCR as examples. An unsuspected signaling pathway for positive selection, and for alloreactivity, is predicted. Further, crucial data needed to elucidate the structural elements that distinguish signaling for restrictive- versus allo-reactivity are identified.

Contemplating Bretscher's View that the Tritope Model is 'Implausible'

There is, at present, only two models of the TCR structure-function relationship. These are referred to here as the Standard (Centric) model and the Tritope model. While I have argued that the Standard model is untenable and proposed the Tritope to replace it, Bretscher has argued that the Tritope model is 'implausible' and throws his support for the Standard model. This essay analyses the implausibility argument concluding that it is unfounded.

Is the Framework of Cohn's 'Tritope Model' for How T Cell Receptors Recognize Peptide/Self-MHC Complexes and Allo-MHC Plausible?

Cohn has developed the tritope model to describe how distinct domains of the T cell receptor (TcR) recognize peptide/self-MHC complexes and allo-MHC. He has over the years employed this model as a framework for considering how the TcR might mediate various signals [1-5]. In a recent publication [5], Cohn employs the Tritope Model to propose a detailed mechanism for the T cell receptor's involvement in positive thymic selection [5]. During a review of this proposal, I became uneasy over the plausibility of the underlying framework of the Tritope Model. I outline here the evolutionary considerations making me question this framework. I also suggest that the proposed framework underlying the Tritope Model makes strong predictions whose validity can most probably be assessed by considering observations reported in the literature.