Introduction.Patients with HL normally undergo a risk-adapted treatment based on PET scans. Regardless of HL treatment high efficiency, (80-90% remission), the search of new prognostic markers has been a continued process in recent years. This is related to the disease recurrence in 25-35% of patients in the first 5 years. The studies show that MDSC expands dramatically with tumor growth and causes a tumor immunosuppressed environment. The aim of this study is to investigate the number of MDSC in HL patients' peripheral blood and its correlation with clinical variables when the diagnosis and outcome first set. Methods .43 primary HL patients, (median age: 30, range: 19-78 years; male: 18, female: 25), were included in the study since April, 2018. 58.2% and 41.8% of patients have been diagnosed with early and advanced stages of HL, respectively, and received the ABVD or BEACOPP (14/esc) as a 1st-line treatment regimen. Peripheral blood mononuclear cells (PBMC) were isolated by ficoll density gradient centrifugation treatment, after 2-3 chemotherapy cycles (interim), and at the end of treatment (EOT). We assessed the E-MDSC count (CD33+CD11b+CD14-HLA-DR-) using the anti-human antibodies purchased from Beckman Coulter (Brea, CA, USA). We have also used a routine protocol of metabolic PET assessed according to Deauville criteria of response. Results.83.7% of patients achieved remission (CR/PR) during the follow-up period (median timeline: 18.9 months). We recorded a disease progression in 5 (11.6%) patients during and after the 1st line therapy, (time to relapse - 8 months). In addition, those patients had a high level of MDSC before treatment, which also remained high after interim and EOT-PET. Median of EFS has not been reached and we continue monitoring all patients. The HL patients who achieved remission had an increased E-MDSC percent (median 12.5%) compared to the age and gender control group, (median 2.03%), and the non-responders' one (median 20%). The number of NK-T (CD3+CD16+CD56+) cells increased by 15 % after EOT compared to pretreatment number, 9.2%, p=0.05. PBMC Evaluation at baseline.The E-MSDC count increases significantly with age: 3.1% vs 12.9% in HL patients under 25 years of age vs over 40, respectively p=0.014. Pretreatment MDSC levels were potentially associated with the disease stage: early stages (1A-2A and 2B) vs advanced stages (12% vs 3.9% vs 15%, respectively p=0.07). Our PET imaging tentatively correlated to the above levels. With that, patients with PET 5DS had a higher number of E-MDSC compared to a healthy control group, while the count is reduced in responders during and after treatment (p=0.06). Theoretically, this means that immunosuppression would be reduced and the antitumor immune response of the body would improve. PBMC Evaluation at interim response assessment.At this step, the age was also confirmed as an independent predicting factor in patients under 25 vs > 40, according to the MDSC level (6% vs 11.5%, respectively p=0.008). The level of E-MDSC was higher in patients who failed to achieve CR/PR (p=0.08). PBMC Evaluation at the EOT:90.6% (39/43) and 9.4% (4/43) of patients had a PET-negative and PET-positivestatus at the EOT, respectively (p < 0.05). We found a strong correlation between the EOT-PET and E-MDSC level. With this, patients with EOT-PET-ve had a lower E-MDSC expression vs EOT-PET+ve (12% vs 42% respectively, p=0.0007), which may potentially become a prognostic factor of response prediction. IPS is an important independent prognostic factor positively correlated to the MDSC count at EOT. In our study, patients in the high-risk group (3-4 IPS) had a significantly higher level of MDSC number than the low-risk group (1-2 IPS) and are more likely not to achieve a positive response to treatment (25% vs 18%, respectively p=0.02). Conclusion.The obtained results suggest that the higher level of MDSC number is associated with unfavorable prognosis of HL, reduce response rate. The challenge of strictly prognostic factor application leads to the research of biomarkers in the HL field. The discovery of new prognostic factors will improve an individualized approach to patient treatment and decrease the disease relapse risk. Disclosures No relevant conflicts of interest to declare.

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