Background Within the hematopoietic compartment, fibromodulin (FMOD) is almost exclusively expressed in chronic lymphocytic leukemia (CLL) lymphocytes. We set out to determine whether FMOD could be of help in diagnosing borderline lymphoproliferative disorders (LPD). Methods We established 3 flow cytometry‐defined groups (CLL [n = 65], borderline LPD [n = 28], broadly defined as those with CLLflow score between 35 and −20 or discordant CD43 and CLLflow, and non‐CLL LPD [n = 40]). FMOD expression levels were determined by standard RT‐PCR in whole‐blood samples. Patients were included regardless of lymphocyte count but with tumor burden ≥40%. Results FMOD expression levels distinguished between CLL (median 98.5, interquartile range [IQR] 37.8–195.1) and non‐CLL LPD (median 0.012, IQR 0.003–0.033) with a sensitivity and specificity of 1. Most borderline LPDs were CD5/CD23/CD200‐positive with no loss of B‐cell antigens and negative or partial expression of CD43. 16/22 patients with available cytogenetic analysis showed trisomy 12. In 25/28 (89%) of these patients, FMOD expression levels fell between CLL and non‐CLL (median 3.58, IQR 1.06–6.21). Discussion This study could suggest that borderline LPDs may constitute a distinct group laying in the biological spectrum of chronic leukemic LPDs. Future studies will have to confirm these results with other biological data. Quantification of FMOD can potentially be of help in the diagnosis of phenotypically complex LPDs.

Plasmacytoid dendritic cells (pDCs) are part of the innate immune system and perform essential functions, such as antigen presenta-

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