Unlocking Predictive Power: Quantitative Assessment of CAR-T Expansion with Digital Droplet Polymerase Chain Reaction (ddPCR).

Flow cytometry (FCM) and quantitative PCR (qPCR) are conventional methods for assessing CAR-T expansion, while digital droplet PCR (ddPCR) is emerging as a promising alternative. We monitored CAR-T transcript expansion in 40 B-NHL patients post-infusion of CAR-T products (axi-cel; tisa-cel; and brexu-cel) with both His-Tag FCM and ddPCR techniques. Sensitivity and predictive capacity for efficacy and safety outcomes of ddPCR were analyzed and compared with FCM. A significant correlation between CAR-T counts determined by FCM and CAR transcripts assessed by ddPCR (p < 0.001) was observed. FCM revealed median CD3+CAR+ cell counts at 7, 14, and 30 days post-infusion with no significant differences. In contrast, ddPCR-measured median copies of CAR-T transcripts demonstrated significant lower copy numbers in tisa-cel recipients compared to the other products at day 7 and day 14. Patients with a peak of CAR transcripts at day 7 exceeding 5000 copies/microg gDNA, termed "good CAR-T expanders", were more likely to achieve a favorable response at 3 months (HR 10.79, 95% CI 1.16-100.42, p = 0.036). Good CAR-T expanders showed superior progression-free survival at 3, 6, and 12 months compared to poor CAR-T expanders (p = 0.088). Those reaching a peak higher than 5000 copies/microg gDNA were more likely to experience severe CRS and ICANS. DdPCR proves to be a practical method for monitoring CAR-T expansion, providing quantitative information that better predicts both treatment outcomes and toxicity.

The CD4/CD8 ratio of infused CD19-CAR-T is a prognostic factor for efficacy and toxicity.

CD4+ and CD8+ chimeric antigen receptor T cells (CAR-T) play different roles in the in vivo anti-tumour response, but the role of the CD4+ /CD8+ ratio among infused CAR-T has not been clearly defined yet. We analysed leftovers from infused anti-CD19 CAR-T bags of 31 patients with aggressive B-cell lymphomas. The median ratio was 1.44, lower for brexu-cel compared to tisa-cel and axi-cel. The CAR+CD4+ /CD8+ ratio was influenced by lactate dehydrogenase levels at apheresis, not by age, previous treatments or the CD4+ /CD8+ ratio in peripheral blood. Patients with a response at 3 months after CAR-T (M3) had a lower CAR+CD4+ /CD8+ ratio in the infused products compared to non-responders (ratio 0.74 vs. 2.47, p = 0.011). A CAR+CD4+ /CD8+ ratio higher than the cut point of 1.12 was associated with an increased risk of treatment failure at M3 (OR 23.3, p = 0.012) and M6 (OR 10, p = 0.028). The median 6-month PFS was 76% for patients with a ratio lower than 1.12% vs. 31% for the others. The prognostic role of the CAR+CD4+ /CD8+ ratio was independent of the costimulatory domain (CD28 vs. 4-1BB) of the product (OR 16.41, p = 0.041). Our data indicate a crucial role for CD8+ CAR-T and the CAR+CD4+ /CD8+ ratio in predicting CAR-T efficacy.