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.