Owing to its recent emergence, COVID\19 has a considerably short history of research. as RBD\specific ASCs. No correlation was observed between the frequency of Bmem cell\derived and spontaneous ASCs, suggesting that the two types of ASCs were weakly associated with each other. Conclusion Our findings reveal that SARS\CoV\2\specific Bmem cells are generated during the acute phase of COVID\19. These findings can serve as a basis for further studies on the longevity of SARS\CoV\2\specific B\cell memory. Keywords: antibody\secreting cell, memory B cell, plasmablast, SARS\CoV\2, virus\neutralising antibodies In this study, we evaluated the frequencies of plasmablasts and memory B cells that specifically target the SARS\CoV\2 receptor binding domain. Our findings reveal that SARS\CoV\2\specific memory B cells are generated during the acute phase of COVID\19. These findings can serve as a basis for further studies on the longevity of SARS\CoV\2\specific B cell memory. Introduction The pathogenesis of coronavirus disease (COVID\19), which has been declared a global pandemic, is PITPNM1 inextricably linked to the immune response to SARS\CoV\2. 1 All three branches of immunity are involved in providing protection against the pathogen. At present, humoral immune response including the production of SARS\CoV\2\specific and virus\neutralising antibodies is the most studied topic related to immunity against COVID\19. 2 There is significant interest regarding antibodies that target the receptor\binding domain (RBD) of the coronavirus surface spike protein, which binds to the target cell by interacting with the entry receptor, the human angiotensin\converting enzyme 2 (ACE2). 3 After binding, the spike protein promotes the entry of the virus into the target cell. The level of RBD\specific antibodies in most patients with COVID\19 exhibits a good correlation with the activity of SARS\CoV\2\neutralising antibodies. This indicates that the RBD is the primary target of SARS\CoV\2\neutralising antibodies, 4 , 5 , 6 thereby suggesting the crucial role of RBD\specific antibodies in infection control. Therefore, the induction of an RBD\specific B\cell response represents an important aspect of immunity against SARS\CoV\2. The prognosis for the further spread of COVID\19 depends on the proportion of recovered patients who are able to acquire a stable immunological memory against SARS\CoV\2 antigens. The quality of the immunological memory will also influence the efficacy of future vaccines for COVID\19. Owing to its recent emergence, COVID\19 has a considerably short history of research. Therefore, long\term predictions for SARS\CoV\2 immunological memory are primarily based on the data currently available for SARS\CoV and other coronaviruses. 7 It has been reported that the level of SARS\CoV\specific IgG peaks during the fourth month, following which it is maintained at a significant level for more than 2?years, 8 after which it gradually declines and tends to reach the baseline level 5C6?years post\disease onset. 9 SARS\CoV\specific memory T\cell response can be detected RO3280 even after 17?years of infection. 10 In contrast, SARS\CoV B\cell memory may not be as long lasting. In a case study on two patients who had recovered from SARS\CoV infection, memory RO3280 B (Bmem) cells could be detected at 3?months post\infection; however, the frequencies were considerably low at 3.5?years after recovery. 9 Information on SARS\CoV\2\specific B\cell memory is considerably limited. Bmem cells obtained from recently recovered patients have been actively used for the isolation and sequencing of immunoglobulin (Ig) genes for the subsequent generation of SARS\CoV\2\neutralising antibodies. 4 , 5 , 11 , 12 However, Bmem cells generation of CD27+CD38+ B cells after 7?days of IL\21/CD40L stimulation in HDs and patients with moderate and severe COVID\19. (d) Representative flow plot of RBD+CD27+CD38+ B cells after IL\21/CD40L stimulation for 7?days; 500?000 B\cell events were acquired. (e) generation of RBD+CD27+CD38+ B cells after IL\21/CD40L stimulation for 7?days. (f) Representative ELISpot showing RBD\specific Bmem cell\derived ASCs. Purified B cells were stimulated with IL\21/CD40L for 7?days and then incubated on ELISpot plates for 16?h to detect cells secreting total (top row) or RBD\specific (bottom row) IgMs (right column) or IgGs (left column). The percentages indicated beside the wells represent the frequencies of antigen\specific ASCs relative to the total RO3280 number of IgMs or IgGs. The wells RO3280 shown contained 104 purified B cells obtained from patients with COVID\19. (g) RBD\specific Bmem cell\derived ASCs per 106 PBMCs in patients with severe (n?=?13) and moderate (n?=?10) COVID\19. (h) Scatter plot of RBD\specific IgG vs. IgM ASCs after 7?days of IL\21/CD40L stimulation. The dotted lines indicate the threshold for a positive RBD\specific ASC response (220 for IgG spots and 1400 for.