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Abstract 4539 Introduction: Dendritic cells (DC) are centrally involved in the initiation of acute graft versus host disease (GVHD) after allogeneic hemopoietic cell transplantation (alloHCT). We have shown that the activation status of peripheral blood CD11c+ myeloid DC, as assessed by CMRF-44 antigen expression, is highly associated with the occurrence and severity of acute GvHD (Transplantation 2007;83: 839–846). However, very little is known about the function of DC after human alloHCT. We examined the relationship between DC functional properties and the severity of acute GvHD. Methods: Peripheral blood CD11c+ myeloid DC from 12 patients were studied weekly up to 8 weeks post transplant for production of IL2, IL4, INFg, IL10 and IL12 using an intracellular cytokine flow assay. Mixed lymphocyte reactions using flow sorted patient DC and third party T cells were used to assess allogeneic immune responses induced by recipient DC in 5 patients. Results: IL12 was the only cytokine detected in post transplant DC. Five of 12 patients developed grade II-IV GvHD, the remaining 7 patients developed either no aGvHD or only grade I. In comparison with pre-transplant levels of expression of IL12, patients with grade II-IV GVHD had a significantly higher percentage of CD11c+ DC expressing IL12 (median 15.1%, range 11.2–20.9%) as compared to patients with grade 0-I GvHD in whom there was no change from baseline values (median 6.6% range 2.8–8.9%) p=0.0025. Increased expression of IL12 was observed in CD11c+ DC commencing at day 25 post transplant. Interestingly, analysis of 5 paired samples comparing sorted DC from normal donors with sorted DC from peripheral blood at day +30 post-transplant showed a marked reduction (measured by thymidine uptake from 60% to 90%) in the capacity of post-transplant donor DCs to stimulate 3rd party lymphocyte proliferation. None of these patients developed clinically significant aGvHD. Conclusion: Production of IL-12p70 by CD11c+ myeloid DC correlates with severity of GvHD despite impaired capacity of these cells to elicit third party proliferative responses. Disclosures: No relevant conflicts of interest to declare.

AIM: Dendritic cells (DC) are considered central to the development of graft versus host disease (GVHD) following allogeneic haematopoietic stem cell transplantation (alloHSCT). Both donor and host DC are thought to initiate allogeneic immune responses by presenting host antigens to donor T lymphocytes. We studied the potential of CMRF-44, a novel monoclonal antibody identifying activated circulating blood DC, as a predictive marker of acute GVHD. METHODS: In a prospective study, peripheral blood was taken from 40 patients twice weekly up to 100 days post-alloHSCT. Circulating myeloid (CD11chi) and plasmacytoid (CD123hi) DCs were enumerated and the expression of CMRF-44 was assessed on CD11chi DC by four colour flow cytometry. Multivariate analyses were performed using a non-parametric Mann-Whitney U-test and Receiver Operating Characteristic (ROC) curves. RESULTS: Following alloHSCT, the severity of acute GVHD correlated with the number of total DC in the blood (p=0.035). Furthermore, low myeloid and plasmacytoid DC numbers were significantly associated with grade 2-4 acute GVHD (p=0.046 and 0.017 respectively). In 40 alloHSCT patients, 27 developed acute GVHD. CMRF-44 was expressed on CD11c+ DC in all cases prior to the onset of acute GVHD. Of the 13 patients without GVHD, 8 had no circulating CMRF-44+ CD11c+ DC. CMRF-44 expression was independent of the reconstitution of myeloid DC (p=0.73). Patients who had CMRF-44+ CD11c+ DC in more than 20% of their post-transplant monitoring samples were more likely to develop acute GVHD (p=0.001, OR=37.1). In addition, patients with more severe grade 2–4 GVHD had significantly higher percentages of CMRF-44+ CD11c+ DCs (p=0.001). CMRF-44 expression at greater than or equal to 12% of CD11chi DCs had a sensitivity of 87.5 for prediction of grade 2–4 acute GVHD, and a specificity of 91.7. CONCLUSION: CMRF-44 expression on blood CD11c+ DC is highly associated with the onset of acute GVHD. These results suggest that CMFR-44 may be used as a predictive tool to identify patients at risk of severe GVHD and to direct therapy. Furthermore, it reinforces the potential application of suitably engineered CMRF-44 antibodies for the prevention or treatment of graft versus host disease.