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clear cells—microglial cells of the astrocytes, in the case of the central nervous system—which in turn produce the oxygen radicals, nitric oxide, etc., that destroy self-tissue.

Over the past 15 years, several groups of researchers have used immune tolerance as a tool for investigating the specificity of immune response in these two models of MS. This line of research began with the serendipitous observation that Th-1-type or delayed-type hypersensitivity responses could be inhibited by injecting naive animals with a population of MHC Class II-bearing APCs that had been incubated with a particular antigen or peptide in the presence of a crosslinking reagent called ethylene carbodiamide (ECDI). Further experiments revealed that this procedure altered the APC in such a way as to block the costimulatory signal 2, leading not to activation but to anergy and/or deletion of the antigen-specific T-cells.

Potential for Peptide Immunotherapy in Relapsing Autoimmune Disease. Building on these findings, researchers discovered that if it was possible to induce EAE by injecting mice with MBP-specific T-cells at day 0, it was also possible to effectively turn off that response by tolerizing the animals anywhere from 7 days prior to 5 days after injection—i.e., prior to initial clinical tissue damage—either with whole MBP (in a mouse spinal cord homogenate or MSCH) or with its immunodominant epitope (in the 84–104 region of the MBP molecule). If they delayed tolerance until after the acute phase of disease, however, it delayed the disease but the animals tolerized with 84–104 eventually relapsed at the same rate as controls, while the crude MSCH (containing not only MBP but every other potential neuroantigen) did a very effective job of inhibiting any further relapses.

The reason for these findings, researchers suspected, was that the initial phase of disease is directed primarily against the immunodominant epitope, but that tissue damage during the acute phase activates responses against endogenous epitopes, a process others have called epitope or determinant spreading. In a more defined experiment, they induced disease with the immunodominant epitope of PLP (amino acids 139–151) and then measured specificity of the responses that followed. The results show that, following the acute phase, there is a clear and consistent response against the secondary 178–191 epitope, a response that was not seen before the acute phase. Researchers also found that 139–151-specific T-cells could be reactivated with the noncrossreactive 178–191 epitope and would then transfer disease to naive animals, suggesting that—although 139–151 is the first self-response to arise—178–191 is primarily responsible for disease relapse. And indeed, tolerizing the animals with either 178 alone or 178 plus 139 protected the animals against further relapses, good evidence that the secondary epitopes have a major role in mediating relapses in this model.

This points to a major problem in using peptide vaccines and peptide therapies: the target keeps changing as time goes on. Other researchers have attempted to circumvent this problem by targeting the B7.1 and B7.2 molecules themselves, and hence inhibiting costimulation. They found that infusing anti-

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