Posted October 23, 2020

Steven Little, Ph.D. University of Pittsburgh
Vijay Gorantla, MD., Ph.D., Wake Forest University

What do cancer and organ transplantation have in common? Initially, it may not seem like much, but both require tricking the body into accepting something foreign as part of itself. Cancer’s ability to evade immune attack contributes to making it such a difficult disease to treat. However, in organ transplantation, recipients require lifelong immunosuppressive drugs in order to minimize and prevent attacks by the immune system that can lead to transplant rejection. Now a team of investigators are trying to harness a strategy used by cancer cells to avoid immune system detection and apply it to transplantation. 

Dr. Steven Little at the University of Pittsburgh, together with Dr. Vijay Gorantla at Wake Forest University, received a Fiscal Year 2014 Reconstructive Transplant Research Program award to evaluate the utility of engineered microparticles (MPs) to promote vascularized composite allotransplant (VCA) acceptance. VCA is the transplantation of multiple tissue types as a functional unit, such as a hand or face; like solid organ transplantation, recipients must adhere to lifelong immunosuppression. In order to promote transplant tolerance without reliance on harsh immunosuppressive drugs, the team has engineered MPs releasing the chemokine CCL22, simulating a strategy used by some tumor cells. Whether secreted by tumors or released from MPs, CCL22 serves to recruit a subset of naturally occurring lymphocytes called regulatory T cells (Tregs). Tregs suppress the effects of other T cells that would ordinarily attack cells that they recognize as foreign; in cancer, this allows tumor cells to avoid destruction by the immune system, but in transplantation, this can lead to immune tolerance with prolonged graft survival in the absence of immunosuppression. 

In a paper published in Science Advances (1), the team demonstrated that two doses of CCL22 releasing MPs, termed “Recruitment-MPs,” can lead to long-term survival of greater than 200 days in a rodent hindlimb transplantation model. Recruitment-MPs were administered via injection into the transplanted hindlimb immediately after surgery, and animals received the immunosuppressive drug tacrolimus for 21 days, at which point it was discontinued and the second dose of Recruitment-MPs was administered. Control animals only receiving 21 days of tacrolimus, without Recruitment-MPs, rejected the hind limb within 2 to 3 weeks after it was discontinued. Treatment with Recruitment-MPs reduced markers of inflammation and increased the percentage of Treg cells in draining lymph nodes of long-term surviving animals. 

Utilizing another mechanism exploited by some tumor cells to avoid immune detection, the team engineered a second set of microparticles to enrich Tregs at the allograft site (2). Tregs make up only ~2-3% of circulating lymphocytes, and this rare population may not be sufficient to resolve an episode of rejection in the setting of VCA transplantation. Engineered Treg-inducing (TRI)-MPs secrete some of the same factors used by tumor cells to promote the differentiation of the more populous naïve CD4+ T cells into Tregs. TRI-MPs were designed as a biodegradable controlled-release system for the delivery of TGF-β1 and IL-2 for inducing Tregs from CD4+ T cells, together with the immunosuppressive drug rapamycin. In a rodent hindlimb transplantation model, recipients received standard immunosuppression for 21 days, and rejection was observed within 2-3 weeks after immunosuppression was discontinued (Figure 1). Those recipients who received two doses of TRI-MPs, injected into the transplanted hindlimb immediately after transplantation and at day 21, survived greater than 300 days with no additional treatment (Figure 1). The team also evaluated each component of the TRI-MP system singly and demonstrated that the combination of TGF-β1, IL-2, and rapamycin is required for long-term survival (Figure 1). 

Together, these results demonstrate that synthetic MPs are capable of recruiting and enriching Treg cells to promote long-term immunosuppression-free survival in a rodent model. Next, the team will be exploring these MPs in a clinically relevant large animal VCA model.

Fig. 1. TRI-MP prevents rejection and promotes long-term limb allograft survival in recipients. (A) Representative images of TRI-MP-treated hindlimb allograft (POD > 300) showing no signs of rejection and an actively rejecting untreated control graft (POD = 38). (B) TRI-MP prolongs hindlimb survival (>300 d in 11/12 animals) in the absence of continued systemic immunosuppression (IS) (P < 0.001 for TRI-MP vs. baseline IS, tacrolimus (FK506) and antirat lymphocyte serum (ALS)). Individual components alone (TGF-β1-MP, Rapa-MP, IL-2-MP) did not confer reliable long-term survival (P < 0.05 for TRI-MP vs. Rapa-MP, P < 0.001 for TRI-MP vs. other individual components). 

1. Fisher JD, Zhang W, Balmert SC, Aral AM, Acharya AP, Kulahci Y, Li J, Turnquist HR, Thomson AW, Solari MG, Gorantla VS, Little SR. 2020. In Situ Recruitment of Regulatory T Cells Promotes Donor-Specific Tolerance in Vascularized Composite Allotransplantation. Sci Adv. Mar 13;6(11):eaax8429. doi: 10.1126/sciadv.aax8429. PMID: 32201714; PMCID: PMC7069700. 
2. Fisher JD, Balmert SC, Zhang W, Schweizer R, Schnider JT, Komatsu C, Dong L, Erbas VE, Unadkat JV, Aral AM, Acharya AP, Kulahci Y, Turnquist HR, Thomson AW, Solari MG, Gorantla VS, Little SR. 2019. Treg-Inducing Microparticles Promote Donor-Specific Tolerance in Experimental Vascularized Composite Allotransplantation. Proc Natl Acad Sci U.S.A. Dec 17;116(51):25784-25789. doi: 10.1073/pnas.1910701116. Epub 2019 Dec 2. PMID: 31792185; PMCID: PMC6925993. 

Link:

Public and Technical Abstracts: Regulatory T Cell-Enriching Microparticles for Promoting Vascularized Composite Allotransplant Survival  

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