Monitoring Memory B Cells by Next-Generation ImmunoSpot Provides Insights into Humoral Immunity That Measurements of Circulating Antibodies Do Not Reveal

Memory B cells (B) provide the second wall of adaptive humoral host defense upon specific antigen rechallenge when the first wall, consisting of preformed antibodies originating from a preceding antibody response, fails. This is the case, as recently experienced with SARS-CoV-2 infections and previously with seasonal influenza, when levels of neutralizing antibodies decline or when variant viruses arise that evade such. While in these instances, reinfection can occur, in both scenarios, the rapid engagement of preexisting B into the recall response can still confer immune protection. B are known to play a critical role in host defense, yet their assessment has not become part of the standard immune monitoring repertoire. Here we describe a new generation of B cell ELISPOT/FluoroSpot (collectively ImmunoSpot) approaches suited to dissect, at single-cell resolution, the B repertoire ex vivo, revealing its immunoglobulin class/subclass utilization, and its affinity distribution for the original, and for variant viruses/antigens. Because such comprehensive B cell ImmunoSpot tests can be performed with minimal cell material, are scalable, and robust, they promise to be well-suited for routine immune monitoring.