Overcoming Skin Immunogenicity in Transplantation Tolerance
Mario G. Solari MD, Kia M. McLean MD, Justin M. Sacks MD, Theresa Hautz, Jignesh V.
Unadkat MD, Elaine K. Horibe MD, Angus W. Thomson PhD, DSc, W.P. Andrew Lee MD
Introduction: Skin is the most immunogenic component of a composite tissue allograft (CTA).
Clinically this has manifested as multiple acute skin rejection episodes in most of the human CTA
performed to date. Intravenous steroids and increased systemic immunosuppression have been
used to mitigate these rejection episodes. These drugs cause direct organ toxicity and are
associated with metabolic dysfunction, opportunistic infection, and malignancy. Topical
immunotherapy is an attractive and practical therapeutic option to provide local
immunosuppression with minimal systemic toxicity. Topical tacrolimus is currently FDA approved
to treat atopic dermatitis, which is indistinguishable from acute rejection histologically. Atopic
dermatitis is characterized by a high proportion of antigen presenting cells (APC) that exhibit high
stimulatory activity toward autologous T cells. This activity is strongly reduced after the
application of topical tacrolimus, as is the number of infiltrating immune cells and overall
inflammatory cytokine production. The present study applies these properties of topical tacrolimus
to CTA. It investigates the potential of topical tacrolimus to maintain a CTA after total withdrawal
of a short course of systemic therapy.
Methods: Wistar Furth to Lewis (full MHC mismatch) orthotopic hind limb transplants were
performed. Groups included: I- topical tacrolimus alone, II- anti-lymphocyte serum (ALS) (0.5mL
x2 doses) + 21 days cyclosporine (CsA) (10/mg/kg/day), III- ALS (2 doses) + 21 days CsA +
topical tacrolimus once daily. The endpoint of the study is grade 3 rejection, defined by
epidermolysis, or 100 days (long term survival). Biopsies (Fig 1&2) of skin, muscle, and bone
were taken for immunohistochemistry and H&E.
Results: All animals in Group I (n=7) developed grade 3 clinical rejection by postoperative day
(POD) 9, similar to controls without treatment. The mean onset of grade 3 rejection was POD 40
with a range of 34-44 in Group II (n=7). In Group III (n=6), two animals developed grade 3
rejection on POD 35 and 56. The remaining 4 experimental animals reached the 100 day
endpoint without grade 3 rejection (Fig 3).
Conclusion: This study demonstrates the feasibility of maintaining a CTA on topical tacrolimus
therapy alone after induction therapy. The induction protocol in this model mirrors what is
currently performed clinically where recipients undergo lymphoid depletion before organ
transplantation, followed by systemic immunosuppression. Preoperative depletion of T cells with
ALS, along with a short course of systemic immunosuppression, prevents acute rejection, while
topical tacrolimus may inhibit immune cell activation and multiplication in the skin component of
the CTA. This novel regimen could reduce or eliminate the morbidity associated with systemic
immunosuppression in clinical CTA.
Figure 1. Representative histology from a group 1 animal at grade 3 rejection. a) The muscle is
infiltrated with lymphocytes with perivascular concentration. b) The skin is heavily infiltrated with
lymphocytes and the epidermis absent secondary to epidermolysis.
Figure 2. A representative long-term survivor from group 3 at POD 102. a) The transplanted limb
is in the foreground and near indistinguishable from the native limb in the background. b) muscle
and c) skin are grossly normal with scattered lymphocytes. The epidermis is intact with a healthy
basal keratinocyte layer.
Figure3. Limb allograft survival data from all three groups.
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