P1357: impaired cellular responses against sars-cov-2 after covid-19 infection in allogeneic hematopoietic stem cell transplant recipients

Background: Allogeneic hematopoietic stem cell transplantation (HSCT) recipents have a higher risk to develop severe forms of COVID-19 after infection with SARS-CoV2 compared to the general population. Aims: To gain further insights into the immune defects leading to increased severity of COVID-19 infection in allogeneic HSCT recipients, we evaluated B and T cells responses in convalescent healthy controls and allogeneic HSCT recipients after COVID-19 infection. Methods: Peripheral blood samples were obtained from allogeneic HSCT recipients (n=11) between 1 and 8 months after COVID-19 infection. Median time of COVID-19 infection since HSCT was 17 months (range 8-70). Healthy controls recruited by the Geneva Blood Transfusion Center 1 to 3 months after mild COVID-19 infection served as controls (n=11). None of the HSCT recipients nor healthy controls has received anti-SARS-Cov2 vaccination before infection. Antibody responses against the SARS-CoV-2 spike protein (anti-S) and nucleocapside phosphoprotein (anti-N) were measured in plasma using the semi-quantitative Elecsys® Anti-SARS-CoV-2 immunoassay (Roche). SARS-CoV-2-specific T cell responses were quantified based on IFN-γ release against a range of peptides from the SARS-CoV-2 Spike protein (S) as well as from the membrane glycoprotein (M) and the nucleocapside phosphoprotein (N) using an Enzyme-Linked ImmunoSpot (ELISpot) assay. SARS-CoV-2- specific T cell clonotypes were identified by genomic DNA T-cell receptor (TCR) sequencing (immunoSEQ® T-MAP™ COVID, Adaptive). Results: We observed no difference in anti-S and anti-N antibody titers between convalescent healthy controls and HSCT recipients (Figure 1A). We observed significantly lower numbers of IFN-γ spot forming units after stimulation with peptides from both the S protein (p=0.0068) and the M plus N proteins (p=0.0067; Figure 1B) in the transplant group as opposed to the control group. Importantly, such differences were not merely due to T cell lymphopenia in HSCT recipients as similar numbers of IFN-γ producing T cells after phytohemagglutinin (PHA) stimulation and similar percentages of CD3+ T cells among PBMCs were measured in PBMCs from HC and HSCT recipients. SARS-CoV-2- specific T cell clonotypes were detectable in both HC and HSCT recipients (Figure 1C). The degree of diversity of the SARS-CoV-2-specific T cell clonotypes differed between the two groups, with HSCT recipients displaying a significantly reduced number of SARS-CoV-2-specific T cell clonotypes compared with HC (p=0.0037; Figure 1D). Such a difference was maintained when T cell clonotypes specific for the S protein (p=0.0011) or the N and M proteins (p=0.036) were analyzed (Figure 1E). As predicted, HSCT recipients displayed a less diverse TCR repertoire compared with HC as revealed by higher Simpson clonality (p= 0.0079). Interestingly, the Simpson clonality negatively correlated with the number of different SARS-CoV-2-specific T cell clonotypes (R2=0.86, p=4.8e−06; Figure 1F). Summary/Conclusion: Our results indicate that allogeneic HSCT recipients display a defect in cellular but not humoral SARS-CoV-2-specific responses after COVID-19 infection. Such impairment was both quantitative, as revealed by reduced IFN-γ release upon stimulation with SARS-CoV-2 peptides, and qualitative as demonstrated by the reduced clonotypic diversity of SARS-CoV-2-specific T cell clonotypes. These findings provide insights into our understanding of post-HSCT immune-dysfunction and into the increased risk of HSCT recipients of developing severe forms of COVID-19 after infection by SARS-CoV-2.