A Novel Model of Pre-Emptive ECP Treatment Shows Significant Reduction of Gvhd-Related Mortality In Mice,
Authors: Florek Mareike , Sega Emanuela I , Mueller Antonia MS , Leveson-Gower Dennis B , Shizuru Judith A , Negrin Robert S 
Published: October 2019

Abstract
Abstract 4025
Graft versus host disease (Gvhd) remains the major cause of morbidity and mortality after bone marrow transplantation (BMT). To prevent Gvhd the graft has to be rendered tolerant to avoid alloreactivity towards the host. One promising approach to induce tolerance in the graft is to administer apoptotic cells to the host. Apoptosis can be induced by extracorporeal photopheresis (ECP), a therapy based on exposure of cells to photoactivable 8-methoxypsoralen (8MPO) and UVA irradiation. The effect of ECP treatment is at least in part, due to alteration of dendritic cell (DC) maturation, leading to reduced antigen presentation and Tcell activation. Moreover, ECP treatment increases the number of regulatory Tcells (Treg), further reducing Tcell activation.
In contrast to clinical practice where ECP is only applied to established Gvhd, we have developed a pre-emptive murine model with the aim of inhibiting the initiation phase of acute Gvhd and improving survival. Here we describe our model and characterization of the mechanisms of action.
Apoptotic host-type splenocytes were infused into MHC matched or mismatched recipients 5 and 2 days prior to lethal irradiation and BMT. Apoptotic cells were generated by incubation with 8-MPO and UVA light (UVAR light set, Therakos). Mice were followed for Gvhd score and survival. ECP-treated mice survived longer both across major (median survival 51 versus 26 days, p<0.0001) and minor (median survival 56.5 versus 9 days, p<0.002) histocompatibility barriers. Importantly, even though ECP treatment promoted tolerance against the host it did not impact the graft versus tumor effect in a B-cell lymphoma model.
To address the mechanisms of this beneficial outcome we investigated Tcell proliferation in vivo (i), ex vivo (ii) and in vitro (iii). (i) Luciferase-expressing Tcon were used to quantify Tcell proliferation in vivo by bioluminescent imaging. ECP-treated mice showed reduced CD4+Tcell proliferation during the initiation phase of Gvhd (day+4) compared to mice receiving Tcon only. (ii) Re-isolation of CFSE labeled Tcon on day+4 after BMT showed a reduction in CD4+Tcell proliferation. (iii) FACS-sorted DC incubated with apoptotic cells for 2 days in vitro showed a diminished capability to induce Tcell proliferation.
To evaluate the impact of ECP treatment on the expression of homing (P-Selectin, α4β7) and activation (CD44, CD69) markers on donor Tcells we performed re-isolation experiments on day+3 and day+7. At day +3 ECP-treated groups showed a reduced expression of these markers in comparison to control groups. These studies demonstrate slower trafficking to Gvhd target organs. Furthermore, at day+7 ECP-treated groups showed increased FoxP3-expressing host Tregs in comparison to control groups whereas donor Treg were similar in both groups. Ongoing experiments will utilize a mouse model in which it is possible to selectively deplete either donor or host Tregs to examine the role of these two subpopulations of Treg in improving Gvhd outcome.
In summary, ECP treatment prior to transplantation led to (i) a significant survival benefit with no impact on graft versus tumor effect; (ii) reduced CD4+ Tcell proliferation; (iii) delayed expression of homing and activation markers during the initiation phase of Gvhd and (iv) an increase in host Treg.
Overall our work suggests that pre-emptive ECP of host cells prior to BMT may be a clinically relevant strategy to prevent acute Gvhd.

Disclosures:
No relevant conflicts of interest to declare.