Long-Term Immunological Consequences of CD19 CAR T Cell Therapy: Analysis of Infectious Complications and Immune Reconstitution
Authors: Riedel Andreas , Weller Jan Frederic , Phely Laurent , Hug Stefan , Faul Christoph , Lengerke Claudia , Bethge Wolfgang Andreas 
Published: December 2024

Introduction
Anti-CD19 chimeric antigen receptor (CAR) T cells have been a major improvement for the treatment of relapsed and refractory B cell neoplasia. Despite its success, the long-term effects and sequela of CAR T cells on the immune system remain poorly understood. CD19 CAR T cells lead to a depletion of B cells, CD19-positive plasma cells and subsequent hypogammaglobulinemia. Furthermore, cytopenia seen after CAR T cell therapy, which is often attributed to an immune effector cell-associated hematotoxicity (ICAHT), aggravates the underlying immune deficiency in these patients. Moreover, this immunosuppressed condition is exacerbated by the previous cytotoxic therapies. As a result, up to 50% of the nonrelapse mortality (NRM) is attributed to infectious complications. Here, we report our results of a one-year follow-up single-center analysis investigating infectious complications and immune reconstitution after CD19 targeted CAR T cell therapy.
Methods
In this retrospective observational study, we analyzed 36 patients with relapsed or refractory Diffuse Large B Cell Lymphoma (DLBCL) receiving CAR T cell infusion in our institution from May 2019 to March 2023. Patients were included in the analysis until progressive disease (PD) or reaching the endpoint of a one-year follow-up. We assessed short-term (&lt;30 days) and long-term (&gt;30 days) infectious complications. Infections were defined as pathological conditions with typical symptoms, supported by laboratory, radiographic or microbiological evidence. Furthermore, kinetics of the immune reconstitution including cell counts, lymphocyte subsets (CD19+, CD4+, CD8+) and levels of immunoglobulins were analyzed.
Results
Short-term infections occurred in almost 50% of the patients. Most of these infectious complications were attributed to bacteria (46%) and were central line-associated (32%) or had a focus in the urinary tract (41%). Unexpectedly, risk analysis for infectious complications revealed a reduction for patients with a high-risk HEMATOTOX score (OR 0.11, CI 0.02-0.59). A correlation between a higher CAR expansion on day 30 and infectious complications was observed (median expansion of all lymphocytes: 1.0% vs. 5.0%, p&lt; 0.02). Furthermore, we detected a change from bacterial towards viral infections (19% vs. 31%) during the long-term follow-up. Moreover, the focus in these patients became the respiratory tract with approximately 47% of all infections. Interestingly, CRS and ICANS grading ≥ 2 seem to be potential risk factors for long-term infections (OR 4.50 CI 1.06-19.11 and OR 4.96 CI 1.01-24.37) in DLBCL patients treated with CD19 CAR T cells. Multivariate analysis did not reveal independent prognostic risk factors.
Median neutrophil counts showed an adequate recovery at day 90, 180 and 360 (1780/µl, 2040/µl and 1900/µl), while absolute lymphocyte counts remained deficient at these time points (740/µl, 580/µl and 735/µl). These results are reflected in the composition of lymphocyte subsets. We observed a deficiency of CD4+ T cells (median: 162/µl, 150/µl and 208/µl) and CD19+ B cells (median: 0/µl) at 90-, 180- and 360-days post-infusion. Median CD8+ T cell counts were 234/µl, 233/µl and 314/µl at 3, 6 and 12 months, respectively. Interestingly, neither the CAR HEMATOTOX risk stratification, the number of prior treatment lines, nor the progression-free survival had a significant influence on the cellular reconstitution within the first year after treatment. Despite frequent preemptive intravenous immunoglobulin substitution in 52% of the patients 6 months after treatment, IgG levels showed deficient values (median: 478.5 mg/dl).
Conclusion
Our results underline the hematological short- and long-term toxicity of CD19 CAR T cells. While bacterial infections seem to appear especially in the first 30 days post-infusion, which may be attributed to the combination of chemotherapy for lymphodepletion and acute hematotoxicity of expanding CAR T cells, the pathogen spectrum changes later to mostly viral infections. These findings are well in line with an accompanied deficient reconstitution of CD4+ T cells, CD19+ B cells, and hypogammaglobulinemia at 6 and 12 months after treatment. Although we could identify CRS and ICANS grading ≥ 2 as risk factors for long-term infections, we believe further and larger studies are needed to characterize patient risk groups and optimize prophylactic approaches in this cohort.