Spike-Specific Memory B Cell Response in Hematopoietic Cell Transplantation Recipients following Multiple mRNA-1273 Vaccinations: A Longitudinal Observational Study

Abstract

Preventing SARS-CoV-2 infection is of utmost importance in allogeneic hematopoietic cell transplantation patients (allo-HCT), given their heightened susceptibility to adverse outcomes associated with SARS-CoV-2 infection. However, limited data are available regarding the immune response to COVID-19 vaccines in these subjects, particularly concerning the generation and persistence of spike-specific memory response. Here, we analyzed the spike-specific memory B cells in a cohort of allo-HCT recipients vaccinated with multiple doses of the mRNA-1273 vaccine and monitored the spike-specific antibody response from baseline up to one month after the fourth dose. After the primary vaccine series, the frequency of spike-specific B cells, detected within the pool of Ig-switched CD19+ cells, significantly increased. The booster dose further induced a significant expansion, reaching up to 0.28% of spike-specific B cells. The kinetics of this expansion were slower in the allo-HCT recipients compared to healthy controls. Spike-specific IgG and ACE2/RBD binding inhibition activity were observed in 80% of the allo-HCT recipients after the first two doses, with a significant increase after the third and fourth booster doses, including in the subjects who did not respond to the primary vaccine series. Additionally, 87% of the allo-HCT recipients exhibited positive cross-inhibition activity against the BA.1 variant. Our findings provide evidence that allo-HCT recipients need repeated doses of the mRNA-1273 vaccine to induceSARS-CoV-2 specific immune response similar to that observed in healthy individuals. This is particularly crucial for vulnerable individuals who may exhibit a limited response to the primary series of SARS-CoV-2 vaccination.

Keywords: SARS-CoV-2; allogeneic hematopoietic cell transplantation recipients; mRNA-based vaccine; spike-specific memory B cells; vaccination.

Figure 1

Schematic representation of the study participants and design. Allogeneic hematopoietic cell transplantation (allo-HCT) recipients (56 subjects) and healthy controls (HCs; 34 subjects) were vaccinated with up to four doses of mRNA-1273 Moderna at day 0, 28, 180, and 360. Blood samples were collected at pre v1 (day 0, baseline), pre v2, +30 v2 (before and 1 month post second dose), pre v3, +30 v3 (before and 1 month post third dose), and pre v4, +30 v4 (before and 1 month post fourth dose). Plasma samples were analyzed for spike-specific IgG and ACE2/RBD inhibition, while peripheral blood mononuclear cells (PBMCs) were examined for spike-specific memory B cell response.

Figure 2

Spike-specific IgG production against SARS-CoV-2 in allo-HCT recipients. (A) The anti-spike IgG titers were assessed over time using ELISA, with sampling conducted at baseline (prior to the first dose, pre v1), before the second dose (pre v2), 30 days after the second dose (+30 v2), before the third dose (pre v3), 30 days after the third dose (+30 v3), before the fourth dose (pre v4), and 30 days after the fourth dose (+30 v4). Data are reported in grey for individual allo-HCT patients and in green the mean value. Healthy subjects were included as controls (HCs), and the mean value is reported in blue. (B) Spike-specific IgG were evaluated in individual allo-HCT recipients at different time points. Data are presented in a box-and-whiskers diagram illustrating the minimum and maximum values of the entire dataset. Antibody end-point titers are indicated as the reciprocal of the sample dilution, reporting double the background OD value. Kruskal–Wallis’ test, followed by Dunn’s post-test for multiple comparisons, was employed to assess statistical differences between groups; ** p ≤ 0.01; *** p ≤ 0.001.

Figure 3

ACE2/RBD binding inhibition following SARS-CoV-2 mRNA vaccination identified in plasma of allo-HCT recipients and HCs. (A) ACE2/RBD inhibition for wild-type SARS-CoV-2 was assessed at +30 v2, +30 v3, and +30 v4. A threshold (indicated by dotted red line) was set at 30% inhibition percentage to differentiate between positive and negative samples. Kruskal–Wallis’ test, followed by Dunn’s post-test for multiple comparisons, was employed to evaluate statistical differences between time points within allo-HCT recipients (green dots) and in HCs (blue dots). Mann–Whitney’s test was used for assessing statistical differences within HC cohort and between the same time points of the two cohorts. * p ≤ 0.05; *** p ≤ 0.001. (B) Comparison of ACE/RBD inhibition for wild-type (WT) and Omicron/BA.1 (BA.1) variants were assessed at +30 v4 in allo-HCT. Mann–Whitney’s test was used for assessing statistical differences between responses against the two variants. *** p ≤ 0.001.

Figure 4

Spike-specific memory B cells. (A) Gating strategy for the identification of S+ IgG–switched B cells. From left to right: B cells were gated as CD19+, CD3− CD14− CD56− cells. Within the B cells gate, spike-specific B cells were defined as Ig-switched B cells (IgM− IgD−) and double-positive for S FITC and S BV421 (S+ B cells). (B) Bar graphs show the percentages of S+ B cells at baseline and 30 days after the second, third, and fourth mRNA-1273 vaccine doses in allo-HCT recipients. (C) Frequencies of S+ B cells at different time points in allo-HCT recipients (green dots) compared to HCs (blue dots). Mann–Whitney’s test, followed by Dunn’s post-test for multiple comparative tests, was used for assessing statistical differences between cell frequency at different time points. * p ≤ 0.05, ** p ≤ 0.01, *** p ≤ 0.001.