The CD69 early activation molecule is overexpressed in human bone marrow mast cells from adults with indolent systemic mast cell disease B EATRIZ D õ ÂAZ -AGUSTõ ÂN, 1 L UIS E SCRIBANO, 1 P ILAR B RAVO, 1 S ONIA H ERRERO, 1 ROSA N UN Ä EZ , 1 R AQUEL NAVA L O  N, 1 L OURDES NAVARRO, 2 A NTONIO T ORRELO, 2 A LBERTO C ANTALAPIEDRA, 1 L UZ DEL C ASTILLO, 3 J ESU  S V ILLARRUBIA , 1 J OSE  L. NAVARRO, 1 J ESU  S F. S AN MIGUEL 4 AND A LBERTO O RFAO 51 Servicio de Hematologõ Âa, Hospital Ramo Ân y Cajal, Universidad de Alcala  de Henares, Madrid, Departamentos de 2 Dermatologõ Âa and 3 Hematologõ Âa, Hospital del Nin Äo Jesu Âs, Madrid, 4 Departamento de Medicina, Servicio de Hematologõ Âa, and 5 Servicio Central de Citometrõ Âa, Centro de Investigacio Ân del Cancer, Universidad de Salamanca, Spain Received 3 January 1999; accepted for publication 13 May 1999 Summary. We have analysed the quantitative expression of surface CD69 antigen on human mast cells (MC), from both normal and pathological bone marrow (BM) samples, using ¯ow cytometry. Our major aim was to analyse whether CD69 is constitutively expressed by normal BMMC and to explore the possible differences between CD69 expression by BMMC from normal controls and patients suffering from different pathological conditions. The constitutive expression of surface CD69 was clearly demonstrated in BMMC; however, systemic mast cell disease (SMCD) patients showed signi®cantly higher levels of surface CD69 expression than healthy controls (P < 0´001), chronic lymphocytic leukaemia (P  0´001), monoclonal gammo- pathy of unknown signi®cance (P < 0´001), multiple mye- loma (P < 0´001) patients, and myelodysplastic syndromes (P  0´002). Furthermore, almost no overlap between the levels of CD69 expression on BMMC was observed between SMCD cases and the remaining groups of individuals except for the paediatric mastocytosis group (P > 0´05). From the other groups of patients, monoclonal gammopathy of unknown signi®cance (P  0´04), myelodysplastic syn- dromes (P  0´03) and paediatric mastocytosis (P  0´003) cases showed a signi®cantly higher expression of surface CD69 as compared to normal subjects. In summary, our ®ndings show that the CD69 antigen is overexpressed in SMCD patients. Keywords: mast cell, indolent systemic mast cell disease, diagnosis, CD69, ¯ow cytometry. CD69 is a type II membrane protein related to a family of natural killer cell activation antigens which is coded on chromosome 12 in humans (Ziegler et al, 1993, 1994; Lopez Cabrera et al, 1993; Schnittger et al, 1993). From the structural point of view, CD69 is a disulphide-linked surface homodimer composed of two chains of 28 and 32 kD (Hara et al, 1986). This molecule includes three domains with different functions: an intracellular, a transmembrane and an extracellular domain. The phosphorylation occurs in the intracellular domain and it is likely to have implications for signal generation; the extracellular domain is a C-type lectin involved in binding to sugars (Testi et al, 1994). Recent studies have shown that CD69 functions as a signal- transmitting receptor in the early phases of cellular activation (Ziegler et al, 1993, 1994; Lopez Cabrera et al, 1993). To date, no physiologic ligand for CD69 has been identi®ed (Cebrian et al, 1988); however, recent studies have suggested that the CD69-ligand molecule could correspond to a carbohydrate-rich cell surface structure (Hamann et al, 1993). The distribution of CD69 has been extensively analysed on mature haemopoietic cells. Accordingly, CD69 is expressed on human resting platelets (Testi et al, 1990) and mediates platelet activation and aggregation (Testi et al, 1990). Moreover, circulating monocytes (De Maria et al, 1994), epidermal Langerhans cells (Bieber et al, 1992) and bone marrow (BM) myeloid precursors (Gavioli et al, 1992) constitutively express CD69. In contrast, resting peripheral British Journal of Haematology , 1999, 106, 400±405 400 q 1999 Blackwell Science Ltd Correspondence: Dr Alberto Orfao, Servicio de Citometrõ Âa, Labor- atorio de Hematologõ Âa, Hospital Clõ Ânico Universitario, Paseo de San Vicente 58-182, 37007 Salamanca, Spain.