Altered Major Histocompatibility Complex Proteins and Peptides for the Induction of Tolerance After Organ Transplantation

Transplantation tolerance is defined as permanent acceptance of an allograft without the need for nonspecific immunosuppressants, which predispose patients to infectious and neoplastic complications. Our approach uses chemically modified antigenic proteins to modify the first signal that triggers allorecognition. The first signal, which is generated by the trimolecular interaction between the T cell receptor, the antigen-presenting major histocompatibility complex (MHC) protein and the antigenic peptide, is distinguished from a second signal that results from the co-stimulatory interactions between T lymphocytes and antigen-presenting cells and the third signal that results from the stimulatory effects of cytokines. Previous studies in animal models have utilised pretransplant inoculation of the recipient with various types of donor-type cells, such as erythrocyte, bone marrow, transfectant or transgenic cells, or extracted transplantation antigens, which have been prepared by sonication, autolysis/proteolysis, detergent treatment or salt (3 mol/L KCl) extraction, which appears to be the most efficient method. Antigens extracted from natural cells induce tolerance in animal models when administered in pretreatment regimens via the intrathymic route (in conjunction with T cell depletion) or via the intravenous route (after preconditioning by total lymphoid irradiation). Exposure to synthetic allopeptides representing sequences from the hypervariable or the constant regions of either class I or class II MHC molecules produces variable effects on in vivo and in vitro alloimmune reactions. In addition to the interactions between peptides and MHC proteins and/or T cell receptor sites, at least some peptides act by binding to receptors of the heat shock protein family, thereby increasing intracellular calcium concentrations without inducing co-stimulatory signals, and/or by interacting directly with unique surface receptors on natural killer cells. However, several factors intrinsic to peptides may limit their use for tolerance induction in vivo. First, peptides are rapidly cleared by non-immunological mechanisms from the circulation, requiring the use of large quantities. Second, peptides are unable to contact the host system in a fashion that reflects the immunogenicity/tolerogenicity of the epitope on the native molecules. Third, peptides are unable to be processed by antigen-presenting cells. Our approach to tolerance induction uses allochimaeric MHC proteins, which are constructed by engrafting selected donor-type tolerogenic epitopes onto host-type MHC molecular backbones. When delivered in rat models via the intrathymic, intraportal or oral gavage route, allochimaeric class I MHC molecules induce immunodominant responses, namely they overwhelm all other responses toward foreign epitopes. Although most peritransplant antigen treatment regimens require concomitant administration of subtherapeutic doses of nonspecific immunosuppressants, some constructs induce tolerance without adjunctive immunosuppressive therapy. One critical requirement for tolerance induction by allochimaeric MHC antigens is the presence of host rather than third-party flanking amino acid sequences. The allochimaeric sequences seem to steer the host response toward tolerance by directly binding to immune cell receptors that deliver a 'self' signal (T cell receptor hypothesis), by directing endosomal catheptic activity to yield tolerogenic rather than immunogenic peptides (peptide hypothesis), or by altering the interactions in the tri-molecular complex, thereby interfering with the usual participation of the MHC molecule on the antigen-presenting cell (supertolerogen hypothesis).