P-009: Cellular dynamics following CAR T cell therapy are associated with response, resistance and cytokine release syndrome in relapsed/refractory myeloma

Introduction: Data on cellular (CAR) T cell dynamics after treatment and the association with response or the occurrence of cytokine release syndrome (CRS) are limited. We performed a longitudinal flow cytometry study to analyze dynamics of CAR T cells and bystander T cells after infusion and correlated findings with CRS and outcome.

Methods: We analyzed 29 patients treated with commercial CAR T cell products (Ide-cel/Cilta-cel). Based on their response on day 30 following CAR T cell infusion, patients were separated into two groups (CR:n=14, no CR:n=15). To assess longitudinal cellular dynamics of (CAR) T cells, peripheral blood was collected at day of re-infusion (d 0) as well as days 7, 14, 30 and 100 after infusion. CAR T cells were detected using a biotin labeled BCMA CAR detection reagent. The T cell compartment was furthermore differentiated by flow cytometry into CD4+/CD8+ CAR T cells and into thymic emigrants, naïve T cells, effector and central memory T cells as well as naïve and memory regulatory T cells.

Results: We found no correlation between infused number of CAR T cells and in vivo expansion of CAR T cells as well as with response and outcome. Peak CAR T cell expansion was detected after two weeks and was predominantly represented by CD8+ cells. Significantly higher numbers of CAR T cells were detected in the first week and first month following infusion in patients in CR (d7: median 63.4/µl vs 7/µl, p=0.046; d30: median 48/µl vs 3/µl, p=0.031). We also observed significant differences between patients in CR as compared to patients with suboptimal response in the non-CAR T cell compartment. While patients not in CR exhibited significantly higher numbers of naïve CD4+ and CD8+ T cells as well as naïve regulatory T cells during the first months after treatment, patients in CR harbored a significantly higher proportion of CD4+ and CD8+ effector memory T cells one moths after treatment (CD4+:58% vs 33% of all CD4+ T cells,p=0.002; CD8+:59% vs 47%,p=0.018). Overall, 17 patients (59%) developed a CRS (grade 1:n=15,grade 2:n=2). Patients developing a CRS showed significantly higher numbers of CAR T cells in the first week (median 104/µl vs 8/µl, p=0.011) and first month (median 19/µl vs 1/µl, p=0.002) following reinfusion. The expansion of CD8+ effector memory T cells two weeks after infusion was significantly correlated to the occurrence of CRS (CRS: median 191/µl vs no CRS: 46/µl, p=0.006). In patients without CRS, significantly higher numbers of central memory CD4+ (median 35/µl vs 12/µl, p=0.007) and CD8+ T cells (median 10/µl versus 3/µl, p=0.019) as well as naïve CD8+ cells (median 1.3/µl vs 0.1/µl, p=0.021) were observed during the first week.

Conclusions: Longitudinal analyses of CAR T cells and non-transduced T cells show that their numbers correlate with outcome and toxicity of CAR T cell therapy in patients with multiple myeloma. Our study identified patients at risk for relapse and CRS based on the post-treatment cellular dynamics.