Disparity between levels of anti-RBD IgG and anti-nucleocapsid protein IgG antibodies in COVID-19–recovered patients who received a kidney transplant

To the editor:

We read with great interest the study by Chavarot et al.,¹ which demonstrated that anti–severe acute respiratory syndrome coronavirus disease 2 (SARS-CoV-2) anti-nucleocapsid (N) protein IgG declines rapidly following SARS-CoV-2 infection in patients who received a kidney transplant (KTx-pts), independent of illness severity, but did not address the dynamic interplay with IgG antibodies against the spike protein–receptor binding domain (spike-RBD).

We studied 25 KTx-pts and 23 normal control patients, all of whom had nasopharyngeal coronavirus disease 2019 (COVID-19) confirmed by reverse transcription polymerase chain reaction (1 control patient confirmed by blood enzyme-linked immunosorbent assay [ELISA]) and subsequently tested negative for SARS-CoV-2 and recovered. All patients had stable engraftments for an average of 18.6 months (1–52 months) at the time of viral infection.

We used multiplexed microsphere-based assays for the detection of IgG antibodies against the viral N protein and spike-RBD. Most KTx-pts (22 of 25; 88%) were positive for anti–spike-RBD IgG antibodies, and only 28% were positive for anti-N IgG antibodies (Figure 1 a and Supplementary Table S1). All 23 controls developed both anti-N and anti-RBD IgG antibodies (Supplementary Figure S1A). In a subgroup of KTx-pts (n = 12), in which both age (54.5 years old) and infection time (35.8 days) were comparable to a subgroup of control patients (n = 16), we found that although the levels of anti-RBD IgG antibodies were very heterogenous, they were not statistically different from those of normal control patients (P = 0.60). Levels of anti-N IgG antibodies in patients who received a transplant, on the other hand, were significantly reduced, when compared to those of the control patients (P = 0.0022) or compared to the level of anti-RBD IgG in the same group of patients who received a transplant (P = 0.0449). This result (Supplementary Figure S1B) suggests that anti-N IgG antibodies, but not anti-RBD IgG antibodies, were predominately affected in KTx-pts.

Figure 1.

(a) Detection of anti-nucleocapsid (N) IgG and anti–receptor binding domain (RBD) IgG antibodies in coronavirus disease 2019 (COVID-19)–recovered kidney transplant patients (KTx-pts). A total of 25 severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)–positive patients were tested by multiplexed microsphere-based SARS-CoV-2 IgG assays (Luminex Corps). Results are shown as median fluorescence intensity (MFI). The positive threshold (700 MFI) is represented by a horizontal line for both IgG antibodies; each patient is represented by a vertical line. Scatter plot analysis elucidates the level of both types of IgG antibodies at the time (days) of post-COVID confirmation for each patient. (b) Longitudinal analysis of anti-viral IgG antibodies in 6 KTx-pts. Samples were analyzed for the presence of anti-N IgG and anti-RBD IgG antibodies after SARS-CoV-2 infection at different time points. For each patient, a sample taken prior to their exposure to SARS-CoV-2 (before February 2020) was used as an internal control, with a value typically <100 MFI. Day 0 is designated as the day the SARS-CoV-2 infection was confirmed. Antibody positivity was set as previously described. Except for 1 patient (case 14) who had underdetectable (656 MFI) anti-N IgG antibodies on a pre-pandemic date, no other patients had pre-existing anti-N or anti-RBD IgG antibodies.

Longitudinal analyses of anti-N and anti–spike-RBD antibodies were studied in 6 KTx-pts with multiple sera samples available. Figure 1b shows heterogenous yet rapid induction of anti-RBD IgG antibodies, with persistence for at least 100 days (median fluorescence intensity [MFI] >700) and still present at 200 days in 4 patients. However, significantly lower levels of anti-N antibodies were produced, and by day 100, only 1 patient had anti-N IgG antibodies. This patient was noted to have pre-existing anti-N IgG antibodies detected 138 days prior to SARS-CoV-2 infection, consistent with previous exposure to another type of epitope–sharing corona viruses.²

Even with different methodologies, our results are consistent with the findings of Chavarot et al. ¹ that anti-N protein IgG was induced in KTs-pts and that these antibodies rapidly decline over time. SARS-CoV-2 N protein shares a high degree of amino acid identity with SARS-CoV (90%) and Middle East respiratory syndrome (MERS)–CoV (45%). The role of immunodominant anti-N IgG antibodies in providing protective antiviral immunity is currently unknown. By comparing levels of anti-N antibodies with those of concurrent anti-RBD antibodies over a prolonged period in patients who received a transplant, our findings provide a rare opportunity to look into the immunologic dynamics of these individuals, further extending current understanding of the anti–SARS-CoV-2 immune response in immunocompromised patients. In summary, our results clearly demonstrate a disparity between the levels of anti-N and anti-RBD IgG antibodies in COVID-19–recovered post-transplant patients. Our findings of persistence of anti-RBD IgG antibodies suggest that patients who recovered from a transplant may have developed long-lasting anti-RBD IgG antibodies, with a potential neutralizing effect against common strains³ or some of the new SARS-CoV-2 variants.⁴ Larger studies are needed to estimate the degree of acquired protection against reinfection.