Costimulation blockade inhibits the indirect pathway of allorecognition in nerve allograft rejection

Abstract

Nerve allografts provide a temporary scaffold for host nerve regeneration. The need for systemic immunosuppression limits clinical application. Characterization of the immunological mechanisms that induce immune hyporesponsiveness may provide a basis for optimizing immunomodulating regimens. We utilized wild-type and MHC class II-deficient mice, as both recipients and donors. Host treatment consisted of triple costimulatory blockade. Quantitative assessment was made at 3 weeks using nerve histomorphometry, and muscle testing was performed on a subset of animals at 7 weeks. Nerve allograft rejection occurred as long as either the direct or indirect pathways were functional. Indirect antigen presentation appeared to be more important. Nerve allograft rejection occurs in the absence of a normal direct or indirect immune response but may be more dependent on indirect allorecognition. The indirect pathway is required to induce costimulatory blockade immune hyporesponsiveness.

Figure 1

Flow chart illustrating the direct and indirect pathway for alloantigen recognition. (Adapted by permission from Journal of Neurosurgery; 2010 In press, Ray et al. Effect of Cold Nerve Allograft Preservation on Antigen Presentation and Rejection) I have attached an updated version

Figure 2

As demonstrated the presence of the indirect pathway is necessary for costimulatory blockade induced immune hyporesponsiveness. MHC−/− mice that received BalbC nerve allografts treated with triple costimulatory blockade with or without adoptive transfer of CD8-depleted splenocytes (Groups 7 and 9) demonstrated relatively poor histomorphometric axonal regeneration. Allograft controls (Group 5) treated with costimulatory blockade had significantly lower fiber counts than Wild-type BalbC mice (Group 6) that received MHC−/− nerve grafts treated with costimulatory blockade. Isograft controls were not statistically different than MHC−/− isografts controls.

Figure 3

Histologic specimens (toluidine blue stain, 400X) of nerve graft or distal to nerve graft showing excellent regeneration in A) untreated isograft group, poor regeneration in B) allograft group, and C) Group 6 illustrates robust regeneration, that requires an intact indirect pathway.