were also measured, often decreasing by anywhere from 50 percent to complete clearance. These results are very early, but they do show that—even in the face of chronic antigen load and viral infection—introducing the antigen in a suitably immunogenic form can lead to an immune response to the virus.
While the tetanus toxin (TT) peptide in this vaccine is relatively MHC-unrestricted, there are nevertheless several MHC haplotypes to which it will not bind, in which cases it is not immunogenic. Examples include DR4 and some of the DR2 splits. Researchers had already developed several peptides that were known to bind to most DR alleles, but when they tried to immunize peripheral blood in vitro with a “pan-DR” peptide, they got very little response compared with TT. The reason had to do with the topography of this particular peptide: the MHC contact residues were on a polyalanine background that gave the TCR very little to recognize except the short methyl groups on the side chain of alanine. Researchers substituted lysine and tryptophan for alanine in three positions, giving the TCR more interesting side chains to recognize, and the resulting peptide induces a better proliferative response in vitro than did TT.
Like TT, pan-DR represents a helper epitope that may be very useful in conjunction with CTL epitopes to augment the immune response. It also shows considerable usefulness in helping antibody responses, and it might be an interesting peptide to add to some of the prophylactic vaccines that are currently used to generate carbohydrate antigen antibodies.
In response to questions from the audience, Dr. Grey added the following:
Peripheral blood is admittedly a poor compartment in which to observe CTL response, and these in vitro results may not be predictive.
Normal subjects in the Phase I study were subsequently given traditional surface antigen vaccine and responded normally—decreasing EA, DMA, and surface antigen levels, indicating that the vaccine is in fact affecting viral expression.
Once they are linked to helper epitopes, the peptides no longer bind directly to the MHC Class I molecules on the APCs. However, they are being processed somewhere—intra- or extracellularly—and the linkage does not destroy their ability to prime CD8 cells. Perhaps phagocytic cells are taking up these lipopeptide adjuvants.
The original DR peptides were originally developed as MHC blockers in autoimmune disease. They worked well in vitro, but in vivo they have an extremely short half-life and quickly fall below the concentration needed to demonstrate MHC blockade.
HIV peptide vaccine studies have shown that the CTL epitope and helper or adjuvant must be covalently linked—a mixture doesn’t work. Processing appears to extracellular, and the small size of the peptides may allow them to bypass the Class I processing pathway.
These short peptides pulsed onto dendritic cells have effective properties in vitro, which Dr. Berzofsky will address in the following presentation (see below). Briefly, part of the function of CD4 help in inducing CD8 CTLs is in