A comprehensive assessment of long-term SARS-CoV-2-specific adaptive immune memory in convalescent COVID-19 Solid Organ Transplant recipients

Abstract

Long-term adaptive immune memory has been reported among immunocompetent individuals up to eight months following SARS-CoV-2 infection. However, limited data is available in convalescent patients with a solid organ transplant. To investigate this, we performed a thorough evaluation of adaptive immune memory at different compartments (serological, memory B cells and cytokine [IFN-γ, IL-2, IFN-γ/IL12 and IL-21] producing T cells) specific to SARS-CoV-2 by ELISA and FluoroSpot-based assays in 102 convalescent patients (53 with a solid organ transplants (38 kidney, 5 liver, 5 lung and 5 heart transplant) and 49 immunocompetent controls) with different clinical COVID-19 severity (severe, mild and asymptomatic) beyond six months after infection. While similar detectable memory responses at different immune compartments were detected between those with a solid organ transplant and immunocompetent individuals, these responses were predominantly driven by distinct COVID-19 clinical severities (97.6%, 80.5% and 42.1%, all significantly different, were seropositive; 84% vs 75% vs 35.7%, all significantly different, showed IgG-producing memory B cells and 82.5%, 86.9% and 31.6%, displayed IFN-γ producing T cells; in severe, mild and asymptomatic convalescent patients, respectively). Notably, patients with a solid organ transplant with longer time after transplantation did more likely show detectable long-lasting immune memory, regardless of COVID-19 severity. Thus, our study shows that patients with a solid organ transplant are capable of maintaining long-lasting peripheral immune memory after COVID-19 infection; mainly determined by the degree of infection severity.

Keywords: COVID-19 infection; adaptive immunity; solid organ transplantation.

Graphical abstract

Figure 1

Flowchart of the study. ∗P < 0.05 (χ² test and t test). COVID-19, coronavirus disease 2019; IQR, interquartile range.

Figure 2

Heatmaps generated by hierarchical clustering of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)–specific immune responses for solid organ transplant (SOT) and immunocompetent (IC) patients, according to the coronavirus disease 2019 (COVID-19) disease severity (moderate/severe, mild, or asymptomatic). Immune responses used for clustering were differentially expressed (fold change >2, false discovery rate P < 0.05). Gray fields indicate missing values. IFN, interferon; IL, interleukin.

Figure 3

IgG antibody responses to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike and nucleocapsid proteins. (a) Proportion of solid organ transplants (SOT) and immunocompetent (IC) individuals with detectable IgG antibodies. (b) IgG antibody titters against antigens Spike and nucleoprotein among SOT and IC; ∗P < 0.05. (c,d) Seropositive proportion of patients for spike (c) and nucleoprotein (d), according to infection severity at the onset. In columns, immunosuppression status for every cluster of severity. (e,f) IgG-spike (e) and IgG-nucleoprotein (f) titters according to severity and immunosuppression group; ∗P < 0.05. Detailed data on antibody titters are available in Supplementary Table S3.

Figure 4

Frequencies of receptor binding domain (RBD)–specific IgG-producing memory B cells (mBCs). (a) Proportion of solid organ transplant (SOT) and immunocompetent (IC) individuals with detectable RBD-IgG–producing mBCs. (b) Frequencies of RBD-IgG–producing mBCs between SOT and IC. (c) Proportion of individuals with detectable RBD-IgG–producing mBCs according to infection severity at the onset. In columns, immunosuppression status for each severity group. (d) Frequencies of RBD-IgG–producing mBCs according to severity and immunosuppression group; ∗P < 0.05. Detailed data on ratio of RBD-IgG–producing mBC are provided in Supplementary Table S4.

Figure 5

Proportion of patients with detectable severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)–reactive cytokine-producing T-cell responses according to infection severity. Interferon-γ (IFN-γ), interleukin-2 (IL-2), IFN-γ/IL-2, and IL-21 were assessed. ∗P < 0.05.

Figure 6

Global T-cell responses specific to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2; median T-cell frequencies against the 3 SARS-CoV-2 immunogenic antigens: S, M, and N). Significant intra- and intergroup differences (solid organ transplant [SOT] severe symptoms [SEV], SOT mild symptoms [MILD], SOT asymptomatic [ASYMP], immunocompetent [IC] SEV, IC MILD, and IC ASYMP) are shown; ∗P < 0.05. IFN-γ, interferon-γ; IL-2, interleukin-2; SFU, spot forming unit.

Figure 7

Correlations between serologic and cellular immune compartments against (spike) severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) antigen. IgG titters against antigen S and circulating (receptor binding domain [RBD]–spike)-specific memory B cell (mBc) frequencies exhibited a significant positive correlation (r = 0.355, P = 0.003), which was fundamentally driven by immunocompetent (IC) subjects (r = 0.548, P < 0.001). A similar pattern was observed between spike-specific IgG titers and the different (spike)SARS-CoV-2–reactive cytokine-producing T-cell frequencies but for IL-21 (data not shown) was mainly observed within IC individuals (interferon-γ [IFN-γ]: r = 0.358, P = 0.013; interleukin-2 [IL-2]: r = 0.404, P = 0.005; and IFN-γ/IL-2: r = 0.458, P = 0.001). PBMC, peripheral blood mononuclear cell; SFU, spot forming unit; SOT, solid organ transplant.

Figure 8

Dot plots showing the proportion of subjects with detectable responses at the different immune compartments according to disease severity. Humoral memory (H) + T-cell memory (T) = detectable (receptor binding domain [RBD]–spike)-specific memory B cell (mBC) and/or anti-spike IgG and spike-specific interferon-γ (IFN-γ)–producing T cells. Humoral memory = detectable (RBD-spike)-specific mBC or anti-spike IgG. T-cell memory = detectable spike-specific IFN-γ-producing T cells. None (N): no detectable humoral or cellular immunity. SEVERE group: 80% (H+T), 17.5% (H), 2.5% (T), 0% (N); MILD group: 78.9% (H+T), 7.9% (H), 7.9% (T), 5.3% (N); asymptomatic (ASYMP) group: 26.3% (H+T), 26.3% (H), 5.3% (T), 42.1% (N); P < 0.001.

Figure 9

Kinetics of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) IgG antibodies and T-cell responses in severe coronavirus disease2019 individuals between months 1 and 6 after infection. A total of 35 convalescent patients (21 solid organ transplants [SOTs], 14 immunocompetent [IC]) were longitudinally assessed at 2 time points: T1 = 49 (interquartile range [IQR], 44–53) days and T2 = 201 (IQR, 185–208) days after infection. (a) Quantitative and qualitative antibody (spike and nucleoprotein) responses; IgG titters (UA/ml).(b) T-cell frequencies for interferon-γ (IFN-γ), interleukin-2 (IL-2), IFN-γ/IL-2, and IL-21; T-cell frequencies (spot forming unit [SFU]/2 × 10⁵ peripheral blood mononuclear cell [PBMC]). (c) Proportion of patients with detectable T-cell responses for IFN-γ, IL-2, IFN-γ/IL-2, and IL-21. ∗P < 0.05.

Figure 9

Kinetics of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) IgG antibodies and T-cell responses in severe coronavirus disease2019 individuals between months 1 and 6 after infection. A total of 35 convalescent patients (21 solid organ transplants [SOTs], 14 immunocompetent [IC]) were longitudinally assessed at 2 time points: T1 = 49 (interquartile range [IQR], 44–53) days and T2 = 201 (IQR, 185–208) days after infection. (a) Quantitative and qualitative antibody (spike and nucleoprotein) responses; IgG titters (UA/ml).(b) T-cell frequencies for interferon-γ (IFN-γ), interleukin-2 (IL-2), IFN-γ/IL-2, and IL-21; T-cell frequencies (spot forming unit [SFU]/2 × 10⁵ peripheral blood mononuclear cell [PBMC]). (c) Proportion of patients with detectable T-cell responses for IFN-γ, IL-2, IFN-γ/IL-2, and IL-21. ∗P < 0.05.