The redundancy of the immune costimulation system is evidenced by the number of different pathways that have now been described in both the CD40 and CD28/B7 families10,21
. Although some particular characteristics of each pathway are known, their exact contribution and significance to the alloimmune response for the most part remains unclear. It has been shown that the blockade of either of the two most significant pathways has a marked effect on the immune response and the blockade of both has a synergistic effect to inhibit acute rejection6,8
. However, complete inhibition of the immune response is less consistent. It would follow that the blockade of additional costimulatory pathways would promote further suppression of the immune response. Because of the effect of anti-ICOS on activated T cells, it appears to provide an additional mechanistic action to the blockade of the CD40 and CD28/B7 pathways, and this is borne out in the peripheral nerve allograft model. While the double blockade of the CD40 and CD28/B7 pathways has permitted lasting survival of organ and composite tissue allografts, its effect seemed to be more modest for regeneration through the peripheral nerve allograft. The blockade of a third costimulatory pathway (ICOS) was necessary before axonal regeneration approached that seen with the nerve isograft. A possible explanation is that the immunosuppressive requirement for optimal nerve allograft function is higher than that for allograft survival, presuming that cellular survival per se
is necessary but not entirely sufficient for optimal nerve function. The organ allograft differs in that function is dependent on the mass effect of a number of homogeneous cellular units, while the reserve of functioning cells in the nerve allograft is significantly less and therefore more sensitive to the immune response.
The blockade of additional costimulatory pathways may also be useful in the reduction of the dosage of the primary blocking agents to further reduce morbidity and risk22,23
. Particularly, the role of CD40 in platelet activation is better understood24
and reduction in the dosage of its monoclonal blocking antibody may help to reduce the risk of thromboembolism which has been noted in the non-human primate model25
. There is still much that needs to be understood about how immune costimulation can be manipulated in favor of the allograft. It would be logical that the many costimulation-blocking agents available may be used with the same principles that are exploited when developing regimens of conventional pharmacological immunosuppressive medications to further decrease overall morbidity.
An interesting finding is the discrepancy noted between the ELISPOT and the histomorphometric data in regard to immunosuppressive effect. Based on IFN-γ production, a significant reduction in the host immune response is readily seen with double blockade of the CD40 and CD28/B7 pathways. The short regimen of double and triple costimulation blockade appears to provide equivalent immunosuppression with minimal response seen in in vitro cultures. However, the histomorphometric data of axonal regeneration through the nerve allograft demonstrates a much greater difference between the regimens with the double blockade regimen permitting only half as many regenerating axons as the triple regimen or the isograft. There are two potential explanations for these findings. The first is that histomorphometric analysis of axonal regeneration is simply a more sensitive indicator of the magnitude of the immune response than in vitro cytokine production in response to donor antigen. We have previously demonstrated that the cytokine profile of the immune response to nerve tissue is similar to that of skin with predominantly type 1 T helper cell activation, and unlike that of muscle and bone, which show a type 2 immune deviation that is more favorable to the allograft. While the quantitative ELISPOT assay accurately reflects the status of the immune response, nerve tissue appears to be much more antigenic than thought and may require more profound immunosuppression (like skin) for satisfactory regeneration and function. Both tissue types share an abundance of an immunologically active cell population, namely, the Langerhan cells of skin and Schwann cells in nerves both act as antigen presenting cells which facilitate the immune response. The second explanation is that the costimulation-blocking agents may have some other effect on the neurological system that is yet to be identified and is independent of their immunosuppressive properties. As such, while double costimulation blockade may be equally immunosuppressive to the acute response as triple blockade, the neurological effect may be further enhanced, possibly in a synergistic manner, with the use of multiple agents.
In summary, the blockade of multiple costimulatory pathways appears to be a promising strategy for the peripheral nerve allograft model. The addition of an agent to block the ICOS pathway provides further synergy with the blockade of the CD40 and CD28/B7 pathways, significantly increasing the degree of axonal regeneration to a level equivalent to that seen in an isograft, and while being equally immunosuppressive by in vitro assays. The strategy of costimulation blockade is well suited to the temporary immunosuppressive requirements of the nerve allograft as it provides a period of prolonged unresponsiveness that has been shown to be donor-specific and with a duration that extends well beyond the administration of the last dose. Costimulation-blocking agents are currently in clinical use but in combination with conventional pharmacological medications to permit further reduction in medication dosages and to decrease the well-known risks of non-specific immunosuppression.