Abstract
Introduction: Gene therapies for transfusion-dependent thalassemia (TDT) such as betibeglogene autotemcel (beti-cel) alleviate transfusion dependence. These therapies use single-agent busulfan myeloablative conditioning (Bu-MAC) and CD34+ selected autologous gene-modified hematopoietic stem cells (GM-HSCs) depleted of T cells. Bu-MAC is less immunosuppressive compared to conventional allogeneic HSC transplant (allo-HSCT) conditioning; however, the combined impact of Bu-MAC and CD34+ selection on immune reconstitution is not known. Therefore, most centers use infection surveillance/prophylaxis and revaccination protocols developed for allo-HSCT. These protocols isolate patients for months and incur costs and potential toxicities. Yet, prolonged caution may be unnecessary; pre-clinical animal models and a clinical study of autologous transplant suggest that Bu-MAC may only transiently reduce absolute lymphocyte counts (ALCs). Therefore, we hypothesized that patients with TDT reconstitute immune function sooner after beti-cel than after allo-HSCT.
Methods: We conducted a single-center retrospective cohort study of children and young adults with TDT who received beti-cel post-FDA approval at the Children's Hospital of Philadelphia (CHOP) and consented to the CHOP Bone Marrow Transplant Biorepository. Results reported from the time of beti-cel infusion. Cell counts reported as median and range cells/μL. ALCs were assessed frequently before and after infusion. Clinical lymphocyte subsets, PHA-induced T cell proliferation, immunoglobulin concentrations, and tetanus titers were measured before and at 3-month intervals after infusion or until vaccination thresholds were met. Vaccination thresholds were defined per the CHOP transplant section standard of care: absolute CD4+ T cells ≥ 200 cells/μL, T cell PHA stimulation ≥ 30% of control, IgG > 400 mg/dL (and ≥3 months from IVIG infusion), and evidence of IgA or IgM recovery or presence of switched memory B cells. Tetanus titers considered protective if ≥ 0.1 IU/mL. For deep immune cell profiling, we analyzed Ficoll-separated peripheral blood mononuclear cells from a subset of these patients collected 2-, 6-, and 8-months post infusion with a 31-color spectral flow cytometry panel. Circulating T follicular helper cells (cTfh) reported as percent of non-naïve CD4+ T cells that are CXCR5+. Results: We identified 9 patients eligible for the cohort. None had undergone prior splenectomy. Median ALC nadir was 500 (range: 390-1370) and occurred after approximately 2 weeks. Median primary lymphocyte subset counts recovered to and persisted at or above near-normal levels for age for all patients by 3 months: CD3+ = 893 (438-1856), CD3+, CD4+ = 415 (206-675), CD3+, CD8+ = 485 (178-1157), CD19+ = 287 (120-655), NK cells = 104 (47-348). All patients' T cell PHA stimulation responses and immunoglobulin levels met vaccination thresholds at all timepoints. By 3 months, 8 of 9 patients had near-normal counts of naïve (CD19+, IgD+, CD27-) and unswitched memory (CD19+, IgD+, CD27+) B cells – both associated with generating durable protective vaccine responses. 8 of 9 patients also had protective tetanus titers prior to and retained after infusion, including before revaccination.
All samples analyzed by spectral flow cytometry contained near-normal percentages for age of all primary lymphocyte subsets. cTfh, critical for generating durable protective vaccine responses, were present throughout: median: 14.1%, (5.46-24.0%).
Conclusions: These findings suggest that patients with TDT treated with beti-cel are functionally lymphocyte replete 3 months after Bu-MAC and infusion of CD34+ selected GM-HSCs and remain so thereafter. Three months after infusion, patients possessed sufficient lymphocyte subset counts and PHA-induced T cell proliferation responses suggesting that patients may not require further monitoring or prophylaxis for viral reactivation. Patients also had B and T cell subsets associated with generating protective vaccine responses signaling potential vaccine-readiness at this time. Furthermore, immunoglobulins and protective tetanus titers persisted independent of IVIG suggesting that plasma cell function may be unaffected by Bu-MAC. Together these findings suggest that patients with TDT likely retain protective vaccine titers, which combined with adequate lymphocyte function, may not require prolonged isolation or full revaccination post-GM-HSCT in contrast to patients post-allo-HSCT.
