Distinguishing the tritope from the interaction antigen models

Housset and Malissen have presented us with an extrapolation from the facts of structure to an interpretation of T-cell receptor (TCR) function [1]. They operate in the ‘interaction antigen’ framework, albeit ambiguously, arguing that there are generalizations based on structure that rule out ‘dual recognitive’ models. In their section ‘Back to a “hidden” two receptor model’, Housset and Malissen shrug off the Tritope Model [2] by translating it into a ‘two receptor model.’ Therefore, it necessary to point out to these investigators, that there is a difference between a receptor and a receptacle. The Tritope Model describes a ‘dual (triple actually) receptacle–single receptor’. A single receptor can only deliver a single signal to the cell that is interpretable differently by cells in different stages of differentiation. Housset and Malissen favor the popular ‘single receptacle–dual receptor’ model, under which the TCR, a dual receptor, can deliver two distinct signals to the cell, one for negative selection, the other for positive selection, based on degree or length of occupancy [3,4]. Under this model, the TCR has a singlecombining site, which, like the B-cell receptor (BCR), looks at a ‘single interacting face’ or ‘composite epitope’, to use their language, formed by a meld between peptide and the allele-specific determinant on the MHC-encoded restricting element (R). The following experiment designed in the framework of the Tritope Model will disprove this. I have chosen an experiment involving alloreactivity because that was a major concern of their Review [1]. The key assumption being tested is: the recognition of allele-specific determinants is encoded by single V-gene segments. There are many examples of a TCR restricted to one R-element (R) and alloreactive to another (R) (e.g. Refs [5,6]). In these cases, the T-cell is positively selected by R in an H-2 animal and negatively selected by R in an H-2 animal. The question arises, why are these TCRs not either positively or negatively selected by both R-elements? How can a TCR that recognizes two different allele-specific determinants display this discriminatory behavior? Given the earlier assumption, the V-gene segment, designated here Vax ; encodes recognition of R x and the V-gene segment, designated here as Vb; encodes recognition of R. Predictably, the two TCRs made up of this VaVb pair will show two distinctly different behaviors. One TCR will be restricted to R and be alloreactive to R; the other TCR will be restricted to R and alloreactive to R. This will be referred to as a ‘reciprocal pair’. In both cases positive selection would operate through the V-gene segment encoding recognition of the restricting allele and negative selection would operate through the V-gene segment encoding recognition of the allo-allele*. The structural element determining these two behaviors would have to reside in the (NDNJ)b and/or (NJ)a regions of each TCR. Experimental step 1. Isolate two families of T-cell clones, one family restricted to R and alloreactive to R, the other restricted to R and alloreactive to R. This requires selection for alloreactivity to H-2 in an H-2 animal and to H-2 in an H-2 animal, and isolating the clones that see the same pairs of R-elements, allelic or non-allelic (e.g. in mouse, A and A or K and D). Experimental step 2. Select from both families, independently derived clones that use the same VaVb pairs. It is possible that recognition of a given allele-specific determinant will be encoded by more than one V-gene segment owing to their evolution by duplication and selection. Nevertheless, the frequency of identical pairs of VaVb used by both families should be high. This only completes previous findings [5,6] by isolating predicted reciprocal pairs. Given now two families of TCR clones with identical VaVb domains and inverted restrictive and allospecificities, their differential behavior must be a property of the only region that varies, (NDNJ)b and/or (NJ)a. The difference to be looked for must be consistent; it cannot be ‘random’ as the sequences specifying the anti-peptide repertoire would be. Sequencing a number of these regions from reciprocal pairs would reveal the controlling element. The most probable prediction is that the reading frame of Db controls the reciprocal behavior. There should be no overlap between the Db reading frames expressed when Va encodes the restricting specificity and when Vb encodes it. The isolation of behaviorally reciprocal TCR pairs encoded by identical VaVb-gene segments would demonstrate that single V-gene segments specify recognition of allele-specific determinants, thereby defining a germlineselected anti-R site encoded by them. This would disprove the ‘interaction antigen’ or ‘single receptacle–dual receptor’ model and require a re-interpretation of the X-ray