Diagnostic evaluation of specific antibodies against the SARS-CoV-2 virus is mainly based on spike (S) and nucleocapsid (N) proteins. Despite the critical functions in virus infection and contribution to the pattern of immunodominance in COVID-19, exploitation of the most abundant membrane (M) protein in the SARS-CoV-2 serology tests is minimal. This study investigated the recombinant M protein’s immunoreactivity with the sera from COVID-19 convalescents. In silico designed protein was created from the outer N-terminal part (19 aa) and internal C-terminal tail (101–222 aa) of the M protein (YP_009724393.1) and was recombinantly produced and purified. The designed M protein (16,498.74 Da, pI 8.79) revealed both IgM and IgG reactivity with serum samples from COVID-19 convalescents in Western blot. In ELISA, more than 93% (28/30) of COVID-19 sera were positive for IgM detection, and more than 96% (29/30) were positive for specific IgG detection to M protein. Based on the capacity to provoke an immune response and its strong antigenic properties, as shown here, and the fact that it is also involved in the virion entry into host cells, the M protein of the SARS-CoV-2 virus as a good antigen has the potential in diagnostic purposes and vaccine design.

Thyroglobulin (Tg) is a reliable marker for detecting recurrence in differentiated thyroid cancer (DTC) patients, but frequently occurring Tg antibodies (TgAbs) can hinder accurate measurement. We aimed to develop a preanalytical protocol for precise Tg detection in TgAb presence using the immunoradiometric assay (IRMA) platform. This study involved forty-five patients who underwent IRMA Tg and radioimmunoassay (RIA) TgAb measurements, including two patients monitored for recurrence and one with confirmed recurrence. All three had undetectable Tg levels. We evaluated three preanalytical methods in aiming to separate Tg from TgAbs: buffer only (Protocol 1), micro-spin filters only (Protocol 2), and a combination of both (Protocol 3). All preanalytical protocols showed high concordance with the original test (r = 0.981, 0.985, 0.971, respectively, p < 0.001), regardless of TgAb values. Protocols 1 and 3 yielded higher Tg levels than the original test (p < 0.001), especially in the group with a high TgAb titer. Protocol 1 managed to detect Tg in two patients under follow up with initially unmeasurable Tg and high TgAb titers and in one confirmed recurrent case. Sample pre-processing positively influenced Tg detection in TgAb-positive cases. These preanalytical approaches show promise, but further testing with larger sample sizes and more investigated conditions is warranted.

The pandemic caused by the SARS-CoV-2 virus had a great impact on the population of patients treated with peritoneal dialysis (PD). This study demonstrates the impact of infection and vaccination in 66 patients treated with PD and their outcomes during a 6-month follow-up. This is the first research that has studied the dynamics of anti-SARS-CoV-2 IgG in serum and effluent. In our research, 57.6% of PD patients were vaccinated, predominantly with Sinopharm (81.6%), which was also the most frequently administered vaccine in the Republic of Serbia at the beginning of immunization. During the monitoring period, the level of anti-SARS-CoV-2 IgG antibodies in the PD patients had an increasing trend in serum. In the group of vaccinated patients with PD, anti-SARS-CoV-2 IgG antibodies had an increasing trend in both serum and effluent, in contrast to non-vaccinated patients, where they decreased in effluent regardless of the trend of increase in serum, but statistical significance was not reached. In contrast to vaccinated (immunized) patients who did not acquire infection, the patients who only underwent the COVID-19 infection, but were not immunized, were more prone to reinfection upon the outbreak of a new viral strain, yet without severe clinical presentation and with no need for hospital treatment.

Although SARS-CoV-2 immunity has been widely investigated, the relationship between antibody levels and cellular immune responses following different routes of exposure to SARS-CoV-2 remains not fully revealed. In this study, cellular immunity was assessed using a newly developed whole-blood interferon-gamma release assay (nano-IGRA) based on gold nanoparticle (AuNP)-conjugated SARS-CoV-2 antigens. AuNP-conjugated and soluble antigens were compared regarding interferon gamma (IFN-γ) responses and intra- and interassay reproducibility in 112 individuals with immunity acquired through infection, vaccination, or both. Humoral immunity was evaluated by enzyme-linked immunosorbent assay (ELISA). Overall, 88% of participants were seropositive. AuNP-conjugated Spike (S) and Nucleocapsid (N) SARS-CoV-2 antigens induced significantly higher IFN-γ responses than soluble antigens in all exposure groups (p < 0.001). AuNP conjugation reduced intraassay variability by 62.4% and 61.4% for S and N, respectively, and interassay variability by 71.7% and 68.3%, respectively. Antibody levels correlated significantly with IFN-γ responses to S (ρ = 0.358, p < 0.001) and N (ρ = 0.309, p < 0.001). An interesting finding is that cellular responses were also detected in some seronegative individuals. Nano-IGRA, based on a novel AuNP-based approach, provides antigen-specific cellular immune assessment, combining enhanced IFN-γ responses with improved reproducibility. Further studies should establish diagnostic cut-offs, sensitivity, specificity, and clinical performance.