Analyzing antigen-specific antibody titers is a relatively easy way of checking for a specific immune response. However, these titers typically decrease over time. Thus, the question remains whether immune memory is maintained even once titers have fallen, a question particularly important throughout a pandemic caused by a novel virus.
Two studies from the Karolinska Institute, Sweden, employed ELISpot/FluoroSpot to detect SARS-CoV-2-specific T and memory B cells: Authors of Sherina et al. (2021) looked at immunity 6-8 months after infection and the follow-up study by Marcotte et al. (2021) investigated immunity 15 months after infection. Different kinetics were observed for the antibodies in circulation and the cellular response: Antibody titers peaked at 2-4 weeks after infection, while the highest numbers of SARS-CoV-2-specific T and memory B cells were detected 3-6 months after the infection using ELISpot and FluoroSpot. FluoroSpot Path: SARS-CoV-2 (S1scan+SNMO) Human IFN-γ/IL-2 was sensitive enough to detect persistent virus-specific T cells in most patients 15 months after the infection, although numbers were decreasing after six months. Interestingly, disease severity had no significant impact on the numbers of virus-specific T and memory B cells.
To complement the results on the antigen-specific cellular response, the authors suggest combining the assays used with surface phenotyping, for example, to discriminate between effector and memory T cell responses.