(678e) Biomimetic Sustained Release Systems for Regulating Inflammation in Composite Tissue Transplant Rejection

Millions of Americans sustain debilitating or
devastating tissue injury or loss secondary to trauma, sepsis, cancer, or
congenital defects. In most cases, current prosthetics and reconstructive
surgeries fail to provide optimal results in terms of aesthetic or functional
outcomes.  For those patients,
composite tissue allotransplation (CTA)—encompassing transplantation of
hands/limbs, face, abdominal wall, tongue, and larynx—may have the unique
potential to restore the appearance, anatomy, and function of the damaged
tissue. Although the problem of rejection in CTA is
similar to that encountered in solid organ transplantation, CTA involves one of
the most immunogenic organs in the body (the skin). For this reason,
immunological complications following CTA will be exacerbated. In clinical CTA,
the process of rejection is suppressed by systemic delivery of potent immunosuppressants.
These drugs are associated with a host of deleterious side effects and,
unfortunately, do little to delay the process of chronic rejection. An
altertnaive approach to achieve this goal is to harness the innate homeostatic
mechanisms intrinsic to the immune system. Physiologically, such a steady state is
maintained by suppressive lymphocytes called regulatory T cells (Tregs).  In nature, some cell types utilize a
strategy such as this by releasing a chemokine that recruits Tregs. We sought to
mimic this strategy synthetically to promote immune hypo-responsiveness at the
site of CTA.  Accordingly, our group has recently developed rationally designed controlled release
microparticle (MP) systems (referred to as ChemokineMP) capable of reproducing
such a Treg-recruiting chemokine gradient in vivo, leading to localization of
native Tregs at the site of implantation. 
Furthermore, we have developed other formulations (referred to as
FactorMP) capable of inducing the differentiation and expansion of Tregs from a
patient's own na?ve CD4⁺ T cells, which are present in considerably
greater numbers than peripheral Tregs. Recruited Tregs appear to be capable of
inducing local (not systemic) immune hyporesponsiveness and resolution of
destructive inflammation.  Our data
suggests that local recruitment of Tregs (using our mimetic controlled release
chemokine formulations) may even be capable of prolonging rat hind limb CTA in
the absence of long term systemic immunosuppression. Thus, we hypothesize that biomimetic,
controlled release systems that release key cytokines, immunosuppressive
agents, and chemokines can promote long term graft survival in a preclinical
CTA model. 
ChemokineMP release systems were engineered using a double
emulsion-evaporation method as previously described.  A mathematical model of release was used to design the
system such that linear release of the protein would mimic the gradient
produced by tumors.  Following
fabrication and screening, particles were tested in an allogeneic rat hind limb
transplant model. Microparticles were injected subcutaneously at the time of
transplantation of hind limbs from Brown-Norway to Lewis rats and again on
postoperative day 18.  Treatment
with ChemokineMP (one dose postoperatively and another on day 18) show enhanced
survival rates when compared with blank microparticles (BlankMP) (Figures 1 and
2). Further, it was observed that ChemokineMP treated transplants maintained an
intact epidermal layer with low infiltration of inflammatory cells, whereas
treatment with BlankMP show a loss of the epidermal layer by postoperative day
25 with significant infiltration of inflammatory mono-nuclear cells. The outcomes of this
research have the potential to dramatically impact the field of CTA and reconstructive
transplantation by minimizing the need for the number, dosing and duration of
systemic immunosuppression with associated long-term toxicity. Finally, these
biomimetic therapies will also have broader applications unrelated to CTA such
as autoimmune disorders and solid organ transplantation.

Figure
1: Kaplan-Meier
survival curves using control (Blank-MP) and experimental (CCL22-MP) therapies
in an allogeneic rat hind limb transplant model, based on n=5 animals for each
group.

Figure 2: Long term
surviving (>100 days) Lewis Rat with transplanted Brown- Norway Limb.