Interaction between HIV gp41 fusion peptide and T cell receptor: putting the puzzle pieces back together

With regard to the paper by Bloch et al. (1), the authors address an interesting question concerning the molecular mechanisms of the interaction of the 33 amino acid HIV gp41 fusion peptide (FP1–33) and the T cell receptor (TCR). This peptide has been previously shown by Quintana et al. (2) to inhibit antigen specific T cell activation which makes its interaction with TCR both fundamentally and clinically important. To address this question, the authors used in their biophysical, cell and animal studies, several peptides including FP1–8, FP5–13, FP9–16, FP1–33, as well as the TCR core peptide (CP), a synthetic peptide corresponding to the transmembrane domain (TMD) of the TCRct chain. Surprisingly, while the authors state that they identified the TCR/full-length FP (FP1–33) interactions at the molecular level, detailed experiments with N-terminal region peptides were performed but no experiments were done with any peptide(s) corresponding to the C-terminal half of the FP1–33. The interpretation of the obtained data and their discussion give the reader the impression that the FP1–33/TCR interaction is mostly driven by the interaction motif located in the 5–13 region of the FP1–33 and that the observed inhibitory activity of FP is provided only by this particular interaction. This impression can be misleading. In addition, several experimental results are not in agreement with this hypothesis, and these discrepancies are not addressed and discussed. Thus, following the author's hypothesis that the 7 residue long 5–13 region is the only interaction motif that underlies binding of FP1–33 to the TCR and inhibiting T cell activation, it is difficult to explain why in T cell proliferation studies there is no inhibitory activity observed for FP1–16 at 1 and 5 μg/ml whereas FP1–33 and FP5–13 are similarly active at these concentrations showing ~50–60% of inhibition (ref 1:Figs. 1A, B, and 7). This discrepancy becomes more striking when one considers that at the same concentration expressed (in μg/ml), there is ~2- and 4-fold difference in an amount of peptide molecules in FP1–16 and FP5–13 solutions, respectively, as compared with FP1–33. Thus, at this point, it is reasonable to suggest that FP1–33 has at least two interaction interfaces and that one of them is located in the C-terminal half of the peptide. However, this hypothesis was not considered, and using full-length FP1–33 in further experiments, the authors were stating that they are studying the potential interaction of the TCR TMD with FP1–16 (1). Thus, I feel it is important to point out for the reader that there is another possible molecular mechanism describing the action of HIV gp41 FP (throughout the text the term “FP” is used to designate a full-length FP, or FP1–33). A potential mechanism of action of CP and FP. Within the SCHOOL model, these peptides disrupt the transmembrane interactions between the ligand binding TCRa chain and the CD308 and ζζ signaling subunits which normally maintain the integrity of a functional T cell receptor. This prevents formation of signaling oligomers upon multivalent antigen stimulation, thus inhibiting antigen-specific T cell activation (7, 8). It has been previously shown that FP inhibits antigen-specific T cell activation (2) in a manner surprisingly similar to that of TCR CP (3, 4). As mentioned above, TCR CP represents a sequence of the TCRc TMD which has been shown to be critical for TCR assembly and function (5) and to interact with the TMDs of the signaling CD358 and ζ subunits (6), thus maintaining the integrity of the TCR in resting T cells. TCR TM peptides capable of inhibiting antigen-stimulated TCR-mediated T cell activation were first reported in 1997 (3). Since that time, despite extensive basic and clinical studies of these peptides, the molecular mechanisms of action of these clinically relevant peptides have not been elucidated until 2004 when a newly developed model of immune signaling, the signaling chain ho-mooligomerization (SCHOOL) model, was first introduced (7). Considering the close similarity in patterns of inhibition of T cell activation and immunosuppres-sive activity observed for FP (2) and CP (4), the SCHOOL model reasonably suggests a similar molecular mechanism of action for TCR TM peptides and HIV gp41 FP (7, 8). Primary sequence analysis of these two peptides shows different primary sequences but a similarity in charged or polar residue distribution patterns with two positively charged residues spaced apart by 4 (CP) or 8 (FP) amino acids. For CP, Arg and Lys residues are well-known to mediate the interaction between recognition TCRc subunit and signaling CD358 and ζ subunits (6). Importantly, for FP, both arginines are located in the C-terminal half, suggesting that this sequence could be important for the interaction with the TCR. Figure 1 shows a potential mode of action of CP and FP as proposed by the SCHOOL model. Briefly, the CP and FP compete with the TCRa chain for binding to CD3Ö8 and ζ hetero- and ho-modimers, respectively, thus resulting in transmembrane “disconnection” of the signaling subunits from the remaining receptor complex (Fig. 1). This mechanism of FP action suggests the existence of an interaction interface in the C-terminal half of the peptide. It should be noted that the proposed mechanism is the only mechanism consistent with all experimental and clinical data reported to date for TCR TM peptides and their lipid and/or sugar conjugates (9). Considering the hypothesis of the existence of at least two TCR/FP interaction interfaces, the data obtained with full-length FP1–33 and CP in competition studies (1) can be interpreted differently: there is competition between FP1–33 and CP for binding with the transmembrane domains of CD358 and ζ subunits rather than TCRc (Fig. 1). If this interpretation is valid and the N-terminal half of FP specifically binds to TCRa TMD while its C-terminal half binds to CD35e and ζ TMDs, then this putative bipolar binding might result in specific features of FP-induced inhibition of T cell activation that are different from those induced by CP. In conclusion, investigation of the molecular mechanism underlying inhibitory and immunosuppressive activity of HIV gp41 FP may reveal important potential therapeutic targets; however, further studies are needed to clarify the mechanism of action.