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Memory B cells (MBCs) are key for protection from reinfection. However, it is mechanistically unclear how germinal center (GC) B cells differentiate into MBCs. MYC is transiently induced in cells fated for GC expansion and plasma cell (PC) formation, so-called positively selected GC B cells. We found that these cells coexpressed MYC and MIZ1 (MYC-interacting zinc-finger protein 1 [ZBTB17]). MYC and MIZ1 are transcriptional activators; however, they form a transcriptional repressor complex that represses MIZ1 target genes. Mice lacking MYC–MIZ1 complexes displayed impaired cell cycle entry of positively selected GC B cells and reduced GC B cell expansion and PC formation. Notably, absence of MYC–MIZ1 complexes in positively selected GC B cells led to a gene expression profile alike that of MBCs and increased MBC differentiation. Thus, at the GC positive selection stage, MYC–MIZ1 complexes are required for effective GC expansion and PC formation and to restrict MBC differentiation. We propose that MYC and MIZ1 form a module that regulates GC B cell fate.

ABSTRACTDiminished overall survival rate of non-Hodgkin lymphoma (NHL) patients treated with a combination regimen of rituximab, cyclophosphamide, doxorubicin, vincristine and prednisone (R-CHOP) has been recently linked to recurrent somatic mutations activating FOXO1. Despite of the clinical relevance of this finding, the molecular mechanism driving resistance to R-CHOP therapy remains largely unknown. Herein, we investigated the potential role of FOXO1 in the therapeutic efficacy of rituximab, the only targeted therapy included in the R-CHOP regimen. We found CD20 transcription is negatively regulated by FOXO1 in NHL cell lines and in human lymphoma specimens carrying activating mutations of FOXO1. Furthermore, both the expression of exogenous mutants of FOXO1and the inhibition of AKT led to FOXO1 activation in lymphoma cells, increased binding to MS4A1promoter and diminished CD20 expression levels. In contrast, a disruption of FOXO1 with CRISPR/Cas9 genome-editing (sgFOXO1) resulted in CD20 upregulation, improved the cytotoxicity induced by rituximab and the survival of mice with sgFOXO1 tumors. Accordingly, pharmacological inhibition of FOXO1 activity in primary samples upregulated surface CD20 levels. Importantly, FOXO1 was required for the downregulation of CD20 levels by the clinically tested inhibitors of BTK, SYK, PI3K and AKT. Taken together, these results indicate for the first time that the AKT-unresponsive mutants of FOXO1 are important determinant of cell response to rituximab-induced cytotoxicity, and suggest that the genetic status of FOXO1together with its transcriptional activity need further attention while designing anti-CD20 antibodies based regimens for the therapy of pre-selected lymphomas.

Communication between the nuclear lamina protein lamin-A and the actin cytoskeleton is required for optimal T cell responses.