INTRODUCTION In normal adults, myelopoiesis occurs in the bone marrow, where cells of the medullar stroma, locally produced or extramedullar cytokines, and extracellular matrix, form the haematopoietic bone marrow microenvironment. The inter- action of both stromal and haematopoietic cells with cytokines and the associated extracellular matrix molecules controls the progressive commitment and differentiation of myeloid stem cells into mature blood cells. These controls involve both the maintenance of blood cell homeostasis and the capacity to increase production of one or several cell lines in a coordinated and strictly controlled manner. The controls of myeloid cell production and differentiation are thus necessarily complex and tightly regulated. In contrast to this tight regulation of the bone marrow haematopoietic environment stands the fact that partial or full haematopoiesis can be observed in extramedullar loci, like spleen or liver, in many pathological situations in adult life. This is frequent in diseases where the bone marrow microenvironment is either modified, as in myelofibrosis, or occupied by new cell populations, as in neoplasias involving bones. This phenomenon can be understood as a passive dislodging of haematopoiesis into sites that have had the haematopoietic function in earlier embryonic devel- opment: it thus represents a case of myeloid metaplasia. On the other hand, in chronic tissue inflammatory reactions, a local myeloid proliferation may be observed. In schistoso- miasis, both in humans and in experimental models, pro- liferation of mono-macrophagic, neutrophil, eosinophil and megakaryocytic cell lineages, alone or in combination, have been described, in association with diffuse or granuloma- tous inflammatory reactions to parasites (Borojevic and Carvalho, 1981; Borojevic et al., 1981, 1983, 1989a,b; Clark et al., 1988; El-Cheikh et al., 1991). Since in schis- tosomiasis the general bone marrow function was not ham- pered, this phenomenon has to be understood as an active displacement of myelopoiesis outside the normal myelopoi- 477 Journal of Cell Science 104, 477-484 (1993) Printed in Great Britain © The Company of Biologists Limited 1993 In chronic murine schistosomiasis, extramedullar myelopoiesis was observed, with proliferation of myeloid cells in liver parenchyma and in periovular granulomas. We have studied the question of whether cells obtained from granulomatous connective tissue may act as myelopoietic stroma, supporting long-term myeloid pro- liferation. Primary cell lines (GR) were obtained in vitro from periovular granulomas, induced in mouse livers by Schistosoma mansoni infection. These cells were char- acterized as myofibroblasts, and represent liver con- nective tissue cells involved in fibro-granulomatous reactions. They were able to sustain survival and pro- liferation of the multipotent myeloid cell lines FDC-P1 and DA-1 (dependent on interleukin-3 and/or granulo- cyte-macrophage colony stimulating factor, GM-CSF) without the addition of exogenous growth factors. This stimulation was dependent upon myeloid cell attach- ment to the GR cell layer; GR cell-conditioned medium had no activity. Primary murine skin fibroblasts could not sustain myelopoiesis. The endogenous growth-factor was identified as GM-CSF by neutralization assays with monoclonal antibodies. The stimulation of myelopoiesis occurred also when GR cells had been fixed with glu- tardialdehyde. The observed stimulatory activity was dependent upon heparan sulphate proteoglycans (HSPGs) associated with GR cell membranes. It could be dislodged from the cell layer with heparin or a high salt buffer. Our results indicate a molecular interaction between endogenous growth-factor and HSPGs; this interaction may be responsible for the stabilization and presentation of growth factors in myelopoietic stromas, mediating extramedullar proliferation of myeloid cells in periovular granulomas. Key words: proteoglycan, myelopoiesis, granuloma, schistosomiasis SUMMARY Cell membrane-associated proteoglycans mediate extramedullar myeloid proliferation in granulomatous inflammatory reactions to schistosome eggs Márcio Alvarez-Silva, Luiz-Cláudio F. da Silva and Radovan Borojevic * Departamento de Bioquímica, Instituto de Química, Universidade Federal do Rio de Janeiro, Caixa Postal 68021, 21944 Rio de Janeiro, Brazil * Author for correspondence