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Background Myeloid derived suppresor cells (MDSC) are a heterogeneous population of immature myeloid cells that inhibits antitumor responses by T lymphocytes. High levels of MDSC have been found in the tumor microenvironment, which is correlated with a poor prognosis of the disease. Our purposes are to evaluate the objective response rate (ORR) to test the treatment efficacy and to show the decrease of MDSC in clinical benefit (CB) patients, that includes complete response (CR), partial response (PR) and stabilization of disease (SD), when compared with progression of disease (PD). Methods 58 patients diagnosed of R/R DLBCL who were not high dose chemotherapy and autologous bone narrow transplantation candidates were recruited from Spanish hospitals. The patients progressed to a prior R-CHOP therapy and had ECOG ≤ 1. They were treated with the R2-GDP chemotherapy schedule (rituximab, lenalidomide, gemcitabine, dexamethasone and cisplatin). Three peripheral blood analysis were carried out: basal, cycle 3 (C3) and end of induction (EI) by flow cytometry to determine the MDSC level. Results 30 patients obtained CB response to treatment (PR 11 patients, CR 14 patients and SD 5 patients) and 28 patients obtained PD. A remarkable decrease of MDSC is shown in the Table, except in PD patients. On the other hand, ORR was 69% at C3 (PR 53%+ CR 16%) and 66% at EI (PR 40% + CR 26%). Table . 1267P MDSC levels (cells/uL) in CB (CR, PR, SD) and PD. Av: average; St: Standard Deviation; *: Statistically significant differences (P Response Basal Av (St) C3 Av (St) EI Av (St) CB 19.9 (4.0) 10.0 (3.1) 4.8 (2.1)* CR 18.6 (4.9) 12.4 (3.7) 2.7 (1.1)* PR 13.0 (3.2) 4.5 (0.8) 1.4 (0.5)* SD 35.3 (14.8) 18.8 (11.6) 18.5 (9.2) PD 10.7 (4.0) 8.6 (2.4) 12.2 (2.9) Conclusions The ORR obtained shown the treatment efficacy in R/R DLBCL to stimulate the immune system antitumor response, which explains the decrease of MDSC in all CB groups and a non-important increase in PD patients. Based on these results, MDSC seem to be an important therapeutic target against tumor progression, as well as being a potentially promising marker of response. Clinical trial identification EudraCT number: 2014-001620-29. Legal entity responsible for the study Grupo Espanol para el Tratamiento y Estudio de los Linfomas (GOTEL). Funding Grupo Espanol para el Tratamiento y Estudio de los Linfomas (GOTEL). Disclosure All authors have declared no conflicts of interest.