Background CD19. CAR T-cells for the treatment of relapsed and refractory (r/r) Diffuse Large B-Cell Lymphoma (DLBCL) demonstrated complete responses in 40%-58% of the patients. Recently, others could associate high tumor volume and low CAR T-cell expansion in vivo with poor outcome. We hypothesize, that the expansion and immunphenotype of (CAR) T cells in vivo determine treatment response and depend on patient- and disease associated factors.

Materials and Methods Patients with r/r DLBCL (n=34) were treated with either Axi-cel or Tisa-cel at the University Hospitals of Erlangen and Munich (LMU). The CAR T-cell product and peripheral blood were collected on day 0, 4, 7, 14, 30, 60 and 90 post transfusion. CAR T-cells were detected through flow cytometry utilizing a two step-staining with a biotinylated CD19 protein. Effector:Target (E:T) Ratios were estimated as absolute peak expansion of CAR T-cells (/ul) per tumorvolume (cm³). Responder (R, complete or partial remission) were compared to Non-Responder (NR, stable or progressive disease) according to response assessment with PET-CT three months after transfusion.

Results CAR T-cell expansion peaked between day 7 and day 14 after transfusion with a greater expansion of CD8⁺compared to CD8^- CAR T-cells on day 14 (59.27% vs 37.42%, p=0.021). The ratio of CD8⁺ and CD8^- CAR T-cells did not differ between R and NR, however R exhibited higher E:T ratios of CD3⁺ CAR T-cells compared to NR (20.94 vs 12.81, p=0.015) and an increased E:T ratio of CD8⁺ CAR T-cells correlated with better progression-free survival (p=0.033). Interestingly, high CRP and ferritin levels at baseline were inversely associated with the E:T ratio (p=0.048 and p=0.017). CD3⁺ CAR T-cells of R showed earlier peak expression of PD-1 than NR (day 7 vs day 21). Further, peak expansion of CD3⁺ CAR T-cells correlated with higher PD-1 expression in R but not in NR (p=0.003 vs p=0.12). In addition, R revealed an increased relative frequency of effector memory differentiated CD3⁺ CAR T-cells (CCR7^-CD45RA^-, p=0.02), whereas CAR T-cells in NR showed an increased relative frequency of a naïve phenotype (CCR7⁺CD45RA⁺, p=0.001) on day 7 post infusion.

Conclusions Flow-based immunomonitoring with longitudinal characterization of CAR T-cells demonstrated a correlation of the E:T ratio with treatment response and survival. Increased inflammatory conditions at baseline correlated with diminished E:T ratios. Notably, in R CAR peak expansion was positively associated with higher PD-1 expression suggestive for superior CAR T-cell activation. In addition, greater memory differentiation was associated with efficacy during the time of peak expansion. Multiparameter analysis with other clinical covariates will show, whether CAR T-cell expansion and immunphenotypes can predict patient outcome.

Disclosure Information V. Blumenberg: B. Research Grant (principal investigator, collaborator or consultant and pending grants as well as grants already received); Significant; Novartis, Gilead Sciences, Janssen, BMS/Celgene. S. Völkl: None. G. Busch: None. S. Baumann: None. M. Winkelmann: None. B. Tast: None. D. Nixdorf: None. G. Hänel: None. L. Frölich: None. C. Schmidt: None. R. Jitschin: None. F. Vettermann: None. W. Kunz: None. D. Mougiakakos: None. M. von Bergwelt: None. V. Bücklein: None. A. Mackensen: None. M. Subklewe: None.