Donor Requirements For CD4+CD25+FoxP3+ Regulatory T Cells Capable Of Suppressing CD4+ and CD8+ Conventional T Cell Proliferation and Graft Versus Host Disease

We evaluated the ability of highly purified Treg to suppress proliferation of C57BL/6 (H-2^b) Tcons following exposure to irradiated splenocytes from BALB/C (H-2^d) mice in vitro in a mixed lymphocyte reaction (MLR). Either donor derived C57BL/6 (H-2^b) or third party FVB (H-2^q) Tregs suppressed Tcon proliferation at the Treg/Tcon ratios of 1:2 and 1:4. The same Treg population effectively suppressed different MHC derived Tcons where BALB/C (H-2^d) or FVB (H-2^q, third-party) Tcons were incubated with irradiated splenocytes from C57BL/6 (H-2^b) mice and were effectively suppressed with BALB/C (H-2^d) Tregs. In the MLR, third-party Tregs present the same activation molecule expression patterns as MHC matched Tregs: CTLA4 and LAG3 expression is enhanced after stimulation with interleukin-2 (IL-2) and anti-CD3/CD28 beads, while MHC class II molecule expression is increased after 3-4 days of culture with Tcons and irradiated splenocytes. Furthermore third-party and MHC matched Tregs express the same levels of interleukin-10 (IL-10).

We translated these results to in vivo studies in animal models. In these studies T cell depleted bone marrow (TCD BM) from C57BL/6 (H-2^b) mice was injected into lethally irradiated (total body irradiation, 8 Gy) BALB/C (H-2^d) recipient mice. 2 days later GvHD was induced by injecting luc⁺ donor derived Tcons (1x10⁶/mouse). Using this model GvHD was evaluated following the adoptive transfer of freshly isolated CD4⁺CD25⁺FoxP3⁺ Tregs derived from BALB/C (H-2^d, host type), C57BL/6 (H-2^b, donor type), FVB (H-2^q, third-party) or BALB/B (H-2^b, minor mismatched with the donor, major mismatched with the host) mice at the different Treg/Tcon ratios of 1:1, 1:2 and 1:4. As expected, donor Tregs exerted the strongest dose dependent GvHD protection (p = 0.028), while host Tregs did not improve mouse survival (p = 0.58). Third-party and minor mismatched with the donor Tregs improved mouse survival (third-party and minor mismatched with the donor respectively, p = 0.028 and p = 0.17) but mice had worse GvHD score profiles (both p< 0.001) and could not recover their weight as well as mice treated with donor Tregs (both p< 0.001). In vivoTcon bioluminescent imaging confirmed these results showing a reduced Tcon proliferation in mice treated with donor, third-party and minor mismatched with the donor Tregs, the first exerting the strongest effect (after 6 weeks of observation, p< 0.001).

Our studies indicate that MHC disparities between Tregs and Tcons do not represent an insurmountable barrier for Treg function. In vitro and in vivo data strongly suggest that Tregs can suppress Tcon proliferation without requiring MHC matching. In vivo GvHD prevention efficiency was affected by MHC disparities with donor derived Treg being the most effective, however, third party Treg also resulted in GvHD attenuation. These studies indicate that both donor and third party Treg could be effective in clinical application raising the possibility of screening and banking Treg for use. Further, these studies highlight the need for activation of the Treg on host tissues to effectively suppress conventional T cell proliferation and GvHD induction.

No relevant conflicts of interest to declare.