The regulatory T cell effector molecule fibrinogen‐like protein 2 is necessary for the development of rapamycin‐induced tolerance to fully MHC‐mismatched murine cardiac allografts

Abstract

SummaryTherapies that promote tolerance in solid organ transplantation will improve patient outcomes by eliminating the need for long-term immunosuppression. To investigate mechanisms of rapamycin-induced tolerance, C3H/HeJ mice were heterotopically transplanted with MHCmismatched hearts from BALB/cJ mice and were monitored for rejection after a short course of rapamycin treatment. Mice that had received rapamycin developed tolerance with indefinite graft survival, whereas untreated mice all rejected their grafts within 9 days. In vitro, splenic mononuclear cells from tolerant mice maintained primary CD4+ and CD8 + immune responses to donor antigens consistent with a mechanism that involves active suppression of immune responses. Furthermore, infection with lymphocytic choriomeningitis virus strain WE led to loss of tolerance suggesting that tolerance could be overcome by infection. Rapamycin-induced, donor-specific tolerance was associated with an expansion of regulatory T (Treg) cells in both the spleen and allograft and elevated plasma levels of fibrinogen-like protein 2 (FGL2). Depletion of Treg cells with anti-CD25 (PC61) and treatment with anti-FGL2 antibody both prevented tolerance induction. Tolerant allografts were populated with Treg cells that co-expressed FGL2 and FoxP3, whereas rejecting allografts and syngeneic grafts were nearly devoid of dual-staining cells. We examined the utility of an immunoregulatory gene panel to discriminate between tolerance and rejection. We observed that Treg-associated genes (foxp3, lag3, tgf-b and fgl2) had increased expression and proinflammatory genes (ifn-c and gzmb) had decreased expression in tolerant compared with rejecting allografts. Taken together, these data strongly suggest that Treg cells expressing FGL2 mediate rapamycin-induced tolerance. Furthermore, a gene biomarker panel that includes fgl2 can distinguish between rejecting and tolerant grafts.

Overexpression of Fibrinogen-Like Protein 2 Promotes Tolerance in a Fully Mismatched Murine Model of Heart Transplantation

“…T and B cells from fgl2‐ deficient mice ( fgl2 −/− ) have enhanced activation, and fgl2 −/− Tregs have reduced suppressive activity . Recently, we found that fgl2 expression was increased in tolerant heart allografts induced by rapamycin and that an antibody to FGL2 prevented the development of tolerance . Here, we show that rFGL2 prevented rejection of fully mismatched heart allografts as long as rFGL2 was administered.…”

TIGIT signaling restores suppressor function of Th1 Tregs

“…For example, upstream inhibition of PI3K by TIGIT results in inhibition of mTOR, and other substrates of mTOR could be involved. It is worth noting that Fgl-2 production by Tregs, which is under the control of TIGIT, was reported as a hallmark of tolerance to allografts in mice treated with the mTOR inhibitor rapamycin (35), further strengthening the connection between TIGIT signaling and PI3K signaling.…”

Inhibition of the Fibrinogen‐Like Protein 2:FcγRIIB/RIII immunosuppressive pathway enhances antiviral T‐cell and B‐cell responses leading to clearance of lymphocytic choriomeningitis virus clone 13

“…Although we have reported that Treg cells are a major source of FGL2, other cells including macrophages and CD8 + T cells can produce FGL2. Hence at this time we cannot unequivocally state that increased FGL2 in the plasma is solely due to production by Treg . We used mice that were genetically deficient in fgl2 to study the effects of FGL2 on both the innate and adaptive antiviral immune response.…”

“…Hence at this time we cannot unequivocally state that increased FGL2 in the plasma is solely due to production by Treg. 22,51,52 We used mice that were genetically deficient in fgl2 to study the effects of FGL2 on both the innate and adaptive antiviral immune response. Post LCMV cl-13 infection, fgl2 À/À mice had increased numbers of macrophages and DC that expressed CD80, CD86 and MHC-II, markers of macrophage and DC activation in comparison with fgl2 +/+ mice.…”