Leukemia Research Vol. 7, No. I. p. 87-95, 1983. 0145-2126/83/010087-08503.00/0 Printed in Great Britain. ~ 1983Pergamon Press Ltd. EXPRESSION OF AN ACCESSORY CELL PHENOTYPE BY HAIRY CELLS DURING LYMPHOCYTE COLONY FORMATION IN AGAR CULTURE* J. P. FARCET,~+ + M. F. GOURDIN,~ U. TESTA,~Ch. ANDRE,§H. JOUAULT§ and F. REYESt$ tLaboratoire de M~decine Exp~rimentale, $INSERM U.91 et §Laboratoire d'H(~matologie, H6pital Henri Mondor, 94010Creteil, France (Received 22 March 1982. Revised 13 July 1982. A ccepted 15 October 1982) Abstract--Human T lymphocytesrequire the cooperation of accessory cells to generate lympho- cyte colonies in agar culture under PHA stimulation. Various hairy cell enriched fractions, as well as normal monocytes, have been found to be able to initiate colony formation by normal lympho- cytes. Leukemic monocytes from CMML patients were also effective, but not the leukemic lymphocytes from CLL patients. The phenotype expressed by HC in agar colonies was further studied using cell surface and enzymaticmarkers. We have concluded that HC in agar culture in the presence of both normal T lymphocytes and PHA lose the B phenotype that they express in vivo and function like an accessorycell in contrast to normal or leukemic B lymphocytes. Key words: Lymphocyte colony formation, accessorycell, hairy cell, mitogen. INTRODUCTION HAIRY CELL leukemia (HCL) is a clinico-pathological entity characterized by the presence of specific mononuclear cells. These leukemic cells display a pleomorphic phenotype with features that belong to different normal hematopoietic lineages (reviewed in [3, 12]). In most cases the hairy cells (HC) synthesize surface and cytoplasmic immunoglobulins of restricted light chain type and lack receptors for sheep red blood cells. These characteri- stics relate the HC to the B-lymphocyte lineage. However the HC express other features that are more typical of the monocyte/macrophage series: adherence, phagocytosis, a peroxidase activity that can be revealed in the endoplasmic reticulum under electron microscopy [13] and a tartrate-resistant acid phosphatase activity. Furthermore a few HCL cases in which the HC display T-cell features have also been reported [4, 16]; fluctuations in T- and B-cell characteristics of the HC may be observed in vivo [5]. Recently, it has been shown that HC in liquid culture in the presence of PHA switch from a S. Ig ÷ E- phenotype t o S. Ig- E ÷ phenotype [11]. This phenomenon requires the presence of T cells in the culture and consists of a true expression of E receptors but not of the T-cell antigens defined by monoclonal antibodies [19]. These results taken together indicate that the phenotype expressed by HC can be largely modified by an environment alteration. We have been looking at the functional behaviour and the cellular characteristics of HC in semi-solid agar cultures during the colony forma- tion by normal T lymphocytes under PHA stimulation. Primary lymphocyte colonies develop in semi-solid agar in the presence of both normal peripheral blood mononuclear cells and PHA. Under the mitogenic stimulation, the cells migrate onto the upper surface of the gel to form multicellular functional units. These units contain a monocyte and a small number of lymphocytes, two or three according to *This study was supported by CRL 81 2 029 and Universit(~Paris XII No. 900 651 007. Abbreviations: HCL, hairy cell leukemia; HC, hair3.'cells; CLL, chronic lymphaticleukemia; CMML, chronic myelomonocyticleukemia; s.lg, c.lg, surface and cytoplasmic immunoglobulin; TRAP, tartrate resistant acid phosphatase; Po, peroxidase; PHA, phytohemagglutinin. Correspondence to: Dr. J. P. Farcet, Unit(~ de Recherche sur les Anemies, INSERM U.91, H6pital Henri Mondor. 94010Creteil. France. 87