Rejection of the nerve allograft must be minimized or prevented along with nerve regeneration during the optimal treatment method for nerve allograft transplantation. Nerve allograft facilitating methods may be divided into four categories such as: allograft treatment methods, major histocompatibility matching, immunosuppression and tolerance-inducing strategies [3].

The aim of nerve allograft pretreatment methods is to reduce the immunogenicity of the allograft. These include: cryopreservation, lyophilization, freeze-thawing, predegeneration, irradiation and nerve storage with University of Wisconsin Storage Solution [3]. Ten percent dimethyl sulfoxide used as a cryoprotectant for the nerve allograft, and these grafts have demonstrated cell viability after being cooled and stored at −196 °C with liquid nitrogen. Cryopreservation is shown to reduce MHC class II molecules of viable Schwann cells after 1 week of cold preservation, but this technique has failed to demonstrate any significant effect in establishing host tolerance, with decreased axonal growth into the graft [40–43].

Lyophilization that involves freeze-drying is another pretreatment method of nerve allograft in order to decrease antigenicity. Only limited success was gained in only short segment grafts with this method. Besides the nerve regeneration potential was poor after lyophilization [8].

Freeze-thawing is another pretreatment method of nerve allograft, which is composed of repeated cycles of freezing and thawing of donor grafts before transplantation. Lack of controls limit the confidence in regeneration potential of these grafts, although decreased host antigenic response was demonstrated in some studies [3, 44].

The nerve allograft is sectioned prior to harvest and subsequent transplantation in predegeneration method. This process initiates Wallerian degeneration and Schwann cell proliferation and removal of myelin debris is presumed to enhance axonal regeneration. This theoretical advantage could not be demonstrated in any experimental studies [45–47].

Irradiation of nerve allograft is another method for pretreatment of nerve allograft. Ultraviolet-B (UVB: region 280–320 nm) or high voltage gamma rays are used in this method of pretreatment, which results in damage to DNA, modification of surface antigens, interference with antigen processing and presentation [48, 49]. Host’s immune responses are decreased by destruction of Schwann cells, but nerve regeneration across the pretreated allograft is also impaired [22]. In an experimental study, it was shown that pretreatment with a single intraportal dose of UVB-irradiated splenocytes specifically induces tolerance to peripheral nerve allografts in rats [50].

University of Wisconsin Storage Solution is supplemented with penicillin-G (200,000 U/L), dexamethasone (10 mg/L), and regular insulin (40 U/L). This solution has been studied for storage of nerve allografts and proven to decrease rejection with the number of viable Schwann cells. Recipients of stored allografts required lower doses of immunosuppression compared with fresh allografts in clinical trials. The optimal period of storage in this solution was shown to be 7 days at 5 °C [3, 44, 51, 52].

Mackinnon et al. demonstrated good histologic and electrophysiologic evidence of nerve regeneration across matched nerve allograft groups. There was significant antigenic recognition with poor evidence of nerve regeneration between MHC unmatched groups in the same study [53].

Prednisolone and Azothioprine combination therapy was the first successful immunosuppressive protocol in the literature for nerve allotransplantation [22]. Cyclosporin A (CsA) became widely used in 1980s. The nerve regeneration across fresh nerve allograft in CsA-immunosuppressed recipients is claimed to be equivalent to autografts in some experimental studies [54–58]. But on the other hand Meirer et al. proved that CsA overall had direct deleterious effects on peripheral nerve regeneration both for motor and sensory recoveries [59].

FK-506, also known as Tacrolimus is another immunosuppressant that ultimately inhibits the activation of T-cell proliferation [60]. Gold et al. was the first to show augmented nerve regeneration provided by FK-506 in peripheral nerves [61]. Since then, the affectivity of FK-506 in the enhancement of nerve regeneration has been confirmed in multiple experimental models including transection [62], crush [63, 64], chronic axotomy [65], isograft [66], and allograft [11, 67]. Additionally FK-506 has been shown to accelerate functional recovery up to 34 % in small and large animal models [62, 68]. The mechanism of immunosuppression of FK-506 is through calcineurin-inhibition [69, 70], but the exact mechanism of its neuroregenerative properties remains unclear [24].

Immunosuppression of peripheral nerve allograft is needed only for the finite time period required for regeneration of proximal host nerve axons through the allograft, and subsequent reestablishment of host-end organ connections. Systemic immunosuppression could be discontinued after the host axons transverse the graft. The clinical protocols of immunosuppression of nerve allotransplantations are summarized in Table 68.1 [3].

Elimination of immunecompetent T-cells, as an induction therapy, may be performed by selective or non-selective T-cell depletion in order to promote tolerance for nerve allografts [3].

Non-selective depletion of T-cells targets not only alloreactive T-cells, but all T-cells. This is achieved by using polyclonal lymphocyte depleting antibodies, such as antilyphocyte globulin (ALG), antithymocye globulin (ATG), or monoclonal lymphocyte-depleting antibodies directed against CD3 (muromonab) and the anti-CD52 mAb Campath-1H Campath® (alemtuzumab) (Genzyme Corporation, Cambridge, MA.) [3, 71].

Immunomodulating protocol of combination of selective anti-αβ T-cell receptor monoclonal antibodies combined with CsA was used by Scharpf et al. for 5 weeks after nerve allograft transplantation. Altered rejection process affording an increase in nerve regeneration was gained with this protocol. Further experimental and clinical studies are required before routine clinical application of this treatment [72].

The principle of co-stimulatory blockade is the inhibition of second signal of T-cell activation and proliferation. Transient blockade of the various co-stimulation pathways (CD154/CD40 pathway, CD28/B7 pathway) using mAb has been used for prevention of acute and chronic nerve rejection [3, 73, 74]. Thymic alloresistance was achieved via blockage of co-stimulatory molecules CD154-DD40 by using anti-CD154 mAb [73]. The second co-stimulatory pathway CD28/B7 may be inhibited by the use of mAb CTLA4Ig (abatacept), which binds to the CD80 and CD86 receptors present on the antigen presenting cells [75]. LEA29Y (belatacept) is a second-generation agent that acts as a selective co-stimulatory blocker. Preclinical primate studies showed that Belatacept had higher affinity for CD80 and CD86 molecules compared to Abatacept, and found to be more effective [76]. Belatacept was also used in a clinical trial in renal transplantation as a primary maintenance immunosuppressant, and it appeared to preserve the renal function. Besides Belatacept has also reduced the rate of chronic allograft nephropathy compared with CsA-treated patients [77, 78].

Although clinical nerve allotransplantation has been performed since the late nineteenth century with questionable clinical success, the clinical outcomes of one of the first successful series was reported by Mckinnon et al. with the selection criteria, immunosuppressive protocol, and postoperative follow up [11]. The clinical outcome of seven patients (four male, and three female) who underwent reconstruction of peripheral nerve gaps with interpositional peripheral nerve allografts was reported. The nerve allografts were stored in University of Wisconsin Storage Solution at 5 °C for 7 days after harvesting within 24 h of death. Immunosuppressive regimen was composed of CsA, azathioprine, and prednisone in the beginning, and later on FK-506 was used instead of CsA. The details of the regimen were reported elsewhere [11]. The mean time of the immunosuppression was 6 months, which changes with the length of the allograft. Rejection of the allograft occurred in one of seven patients 4 months after the surgery, which was misdiagnosed as infection, and was treated with a course of antibiotics at the beginning [11].

The immunosuppressive protocol of composite tissue allotransplantation including nerves as an important functional component of the first successful transplant is summarized in Table 68.2 [79].

The most commonly used induction therapy includes ATGs, FK-506, mycophenolate mofetil (MMF) and steroids. FK-506, MMF, and steroids are the most commonly used agents for the maintenance therapies.

Current studies most often focus on tolerance and chimerism induction protocols. Stem cell supportive therapy and cellular therapeutics may be systemically administered, via intravenous routes or locally administered [80–83].

A valuable alternative to nerve autograft is acellular nerve allografts that contain natural extracellular matrix components and structure without native cells [84–87].