B cells require DOCK8 to elicit and integrate T cell help when antigen is limiting

Dedicator of cytokinesis 8 (DOCK8) immunodeficiency syndrome is characterized by a failure of the germinal center response, a process involving the proliferation and positive selection of antigen-specific B cells. While DOCK8-deficient B cells are recruited into germinal centers, we find that they are arrested at a light-zone stage. They are unable to respond to T cell–dependent survival and selection signals, and consequently differentiate into plasma cells or memory B cells. Although DOCK8-deficient B cells can acquire and present antigen to initiate activation of cognate T cells, integrin upregulation, B–T cell conjugate formation, and costimulation are insufficient for sustained activation of B and T cells when antigen availability is limited. Our findings provide an explanation for the failure of B cell-dependent humoral responses in DOCK8 immunodeficiency syndrome, and offer insights into how the level of available antigen modulates B–T cell interactions necessary for humoral immune responses and immune memory. Overall design: Hen egg lysozyme (HEL)-specific splenic germinal centre (GC) B cells were sorted from mouse splenocytes on day 8 after adoptive transfer of splenic MD4 B cell mixtures and immunization with SRBC-HEL. CD95+ HEL+ GC B cells from 6 spleens (3 from WT group, 3 from DOCK8-deficient group) were hashtagged (HTO labeled), pooled and sorted by flow cytometry into CD45.1+ (WT) or CD45.2+ (WT or DOCK8-deficient) subsets. The CD45.1 and CD45.2 multiplexed samples were processed independently on the 10X scRNASeq platform.