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Eosinophilic gastroenteritis (EGE) is an inflammatory disease that causes symptoms related to dysfunction owing to the high infiltration of eosinophils into the gastrointestinal tissue. We report a case of EGE that developed after severe eosinophilia induced by nivolumab administration. A 53-year-old woman was diagnosed with recurrent neck lymph node metastasis of right tongue cancer and was referred to our hospital for chemotherapy. After the administration of 12 courses of nivolumab, grade 2 diarrhea was observed, and contrast-enhanced computed tomography (CT) revealed intestinal wall edema and stenosis of the small intestine. Symptoms improved with fasting, discontinuation of nivolumab, and administration of a proton pump inhibitor. Biopsy performed using a colon fiberscope revealed EGE. After resuming nivolumab and administering three courses, positron emission tomography/CT confirmed a complete response to the metastatic lesion, and nivolumab was discontinued. After discontinuation, mild eosinophilia persisted, but abdominal symptoms, such as diarrhea, did not recur. No signs of recurrence or metastasis were observed after two years. [ABSTRACT FROM AUTHOR]

CEBPA‐IGH, a fusion gene of the immunoglobulin heavy‐chain locus (IGH) and the CCAAT enhancer‐binding protein α (C/EBPα) gene, is recurrently found in B‐ALL cases and causes aberrant expression of C/EBPα, a master regulator of granulocyte differentiation, in B cells. Forced expression of C/EBPα in B cells was reported to cause loss of B‐cell identity due to the inhibition of Pax5, a master regulator of B‐cell differentiation; however, it is not known whether the same mechanism is applicable for B‐ALL development by CEBPA‐IGH. It is known that a full‐length isoform of C/EBPα, p42, promotes myeloid differentiation, whereas its N‐terminal truncated isoform, p30, inhibits myeloid differentiation through the inhibition of p42; however, the differential role between p42 and p30 in ALL development has not been clarified. In the present study, we examined the effect of the expression of p42 and p30 in B cells by performing RNA‐seq of mRNA from LCL stably transfected with p42 or p30. Unexpectedly, suppression of PAX5 target genes was barely observed. Instead, both isoforms suppressed the target genes of MEF2 family members (MEF2s), other regulators of B‐cell differentiation. Similarly, MEF2s target genes rather than PAX5 target genes were suppressed in CEBP‐IGH‐positive ALL (n = 8) compared with other B‐ALL (n = 315). Furthermore, binding of both isoforms to MEF2s target genes and the reduction of surrounding histone acetylation were observed in ChIP‐qPCR. Our data suggest that the inhibition of MEF2s by C/EBPα plays a role in the development of CEBPA‐IGH‐positive ALL and that both isoforms work co‐operatively to achieve it. [ABSTRACT FROM AUTHOR]