1970. SARS-CoV-2 T-cell specific responses in allogeneic hematopoietic stem cell transplant recipients after second and third dose of BNT162b2 mRNA vaccine

BACKGROUND: Cell-mediated immunity (CMI) after anti-Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV)-2 vaccines has been poorly explored in recipients of allogeneic hematopoietic stem-cell transplantation (HSCT), especially with regard to the 3(rd) dose (booster). Aim of the study was to assess the specific T-cell responses before and after the 3(rd) dose of BNT162b2 mRNA vaccine in a cohort of allogeneic HSCT recipients, and compare it with healthy donors (HD). METHODS: Allogenic HSCT recipients and HD were enrolled before receiving the 3(rd) dose of BNT162b2 mRNA vaccine. Whole blood for T-cell specific responses was collected before (T1) and 8 weeks after (T2) the booster administration. T-cell responses were assessed with an Interferon (IFN)-γ release assay (IGRA), after overnight stimulation of heparin whole blood with pools of lyophilized peptides, covering the immunodominant sequence of the Spike (S) protein. IFN-γ production was assessed with an enzyme linked immunosorbent assay (ELISA). Statistical analysis was performed with GraphPadPrism. RESULTS: 14 HSCT recipients (8M, 6F) and 15 HD (7M, 8F) were enrolled (table 1). Median age was 47 [39-59] and 41 [31-48] years in the HSCT and HD groups, respectively. Time between the vaccine 2(nd) dose and T1 was significantly longer in HD than HSCT recipients (p< .001), while the time between T1 and T2 did not differ between the two groups. SARS-CoV-2 S specific T-cell responses at T1 were inferior in HSCT recipients compared to HD (median IFN-γ production: 463 vs 231 ng/ml, respectively), although the difference did not reach the statistical significance. No differences were observed at T2. In a before-after analysis, SARS-CoV-2 S specific T-cell responses were significantly increased in HSCT recipients at T2 compared to T1 (median IFN-γ production: 267 vs 881 ng/ml, p=0.02) (Figure 1). At T1, 3 HSCT recipients showed very low or no IFN-γ production, while at T2 only 1 patient still had undetectable IFN-γ production after S peptide stimulation. [Figure: see text] [Figure: see text] CONCLUSION: SARS-CoV-2 IGRA represents a useful tool to assess CMI, also in immunocompromised hosts. An additional 3(rd) BNT162b2 mRNA vaccine booster dose seems to enhance CMI in allogenic HSCT recipients. Further studies are needed to evaluate the duration of SARS-CoV-2 CMI in HSCT recipients compared to HD. DISCLOSURES: All Authors: No reported disclosures.