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PRDM1/Blimp‐1 is expressed in human B‐lymphocytes committed to the plasma cell lineage

PRDM1/Blimp‐1 (in human and mouse, respectively) has a central role in determining and shaping the secretory arm of mature B‐cell differentiation. In this study, a mouse monoclonal antibody that recognizes PRDM1 was used to detail its distribution in normal human lymphoid tissue and in lymphoid neoplasms that correspond to different stages of B‐cell differentiation. PRDM1 was expressed in germinal centre blasts that co‐express Pax5, CD19, CD20, and CD10, but not BCL6 or MTA‐3. Pax5 was downregulated and full plasma cell morphology and phenotype were acquired by PRDM1+, nuclear cREL−, pre‐plasma cells upon exit from the germinal centre. Activated extrafollicular B‐cells (CD30+, Pax5+) were largely PRDM1−. PRDM1 was also absent in tissue histiocytes and the majority of resting T‐cells and S‐100+ antigen‐presenting cells. PRDM1 and CD138 were expressed simultaneously in human lymphomas with plasma cell differentiation, but not in marginal zone lymphomas or chronic lymphocytic leukaemias. A minority of diffuse large B‐cell lymphomas expressed PRDM1 and Hodgkin lymphomas were largely PRDM1−. Infiltrating T‐cells in PRDM1− B‐cell lymphomas expressed PRDM1. In conclusion, PRDM1 staining is a reliable and informative assay to define plasma cell commitment and differentiation in human normal and neoplastic B‐cell lineages. Copyright © 2005 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.