Peripheral blood mononuclear cells: a model for the human vitamin D endocrine system in health and disease

Peripheral blood mononuclear cells: a model for the human vitamin D endocrine system in health and disease Ruth Koren a, Amiram Ravid b and Uri A. Liberman ‘S Ii Instituie of Endocrinology and Metabolism, ’ Rogoff Medical Research -Institute and the Felsenstein Medical Research Institute, Beilinson Medical Center, 49100 Petah-Tikw Israel, and “Department of Physiology and Pharmacology, Sackler Medical School, Tel-Alx Uniwrsity, Tel-At’ir, Israel (Accepted 31 October 1991) Key words: Monocyte; 1,25Dihydroxyvitamin D,; Atopy; Osteoporosis; Vitamin D-resistant rickets; Receptor regulation c9 Studies in recent years have provided ample evidence that 1,25_dihydroxyvitamin D, (1,25(OH),D,I, the natural most active metabolite of vitamin D,, plays a role in the regulation of the immune system (Rigby, 1988). Various cells of the immune system (monocytes, macrophages, thymocytes and activated lymphocytes) contain specific receptors for 1,25(OH),D, (Bhalla et al., 1983; Prowedini et al., 1983). Moreover, the hormone is produced by activated monocytes and macrophages (Reichel et al., 1990) and affects, in an autocrine and paracrine fashion, macrophage and lymphocyte function. The production of 1,25(OH),D, is controlled and regulated by cytokines secreted at sites of immune reactions (Koeffler et al., 1985; Reichel et al., 1987; Bryke et al., 1990). Many of the putative immunoregulatory functions of 1,25(OH),D, are discussed at length elsewere and will not be described here (Rigby, 1988; Manolagas et al., 1989). The purpose of the present article is to review studies on the action of 1,25(OH),D, on cells of the immune system that may provide a hitherto non-existent model for the study of the human vitamin D effector system in health and disease. Correspondence to: Uri A. Liberman, Institute of Endocrinology and Metabolism, Beilinson Medical Center, 49100 Petah-Tikva, Israel. In contrast with classical target organs (intestine, kidney and bone), peripheral blood lymphocytes and monocytes are readily accessible and may be obtained in quantities sufficient for functional and biochemical investigations. To validate peripheral blood mononuclear cells (PBMC) as a model target organ, it must be shown that: (a> the cells contain specific receptors for 1,25(OH),D,, and a hormone-regulated biological response which reflects post hormonereceptor binding events; (b) the receptor-effector system for 1,25(OH),D, is under the same control in PBMC as in other hormone responsive organs. The PBMC model system conforms with these requirements. High affinity receptors for 1,25(OH),D, are expressed constitutively in monocytes and are induced in lymphocytes following activation by mitogens. The binding of 1,25(OH),D, to receptors in PBMC results in a dose-dependent inhibition of lymphocyte activation by antigens and by polyclonal mitogenic lectins (Rigby, 1988; Manolagas et al., 1989). The identity of 1,25(OH),D, receptors in PBMC and in other target organs was established by studies on patients with the rare genetic disease of end-organ resistance to 1,25(OH),D, (vitamin D dependent rickets type II, DDII). Patients with this disorder have severe clinical manifestations of vitamin D deficiency as a con- sequence of very low intestinal calcium absorption, despite normal or high serum levels of 1,25(OH),D, (Marx et al., 1984). These clinical features indicate resistance of the intestinal epithelium to the action of 1,25(OH),D,. Fibroblasts grown out of skin biopsies from these patients have served to elucidate, at the cellular and molecular level, the defects underlying the end-organ resistance to 1,25(OH),D, (Liberman et al., 1983; Malloy et al., 1989; Feldman and Malloy, 1990), and showed a heterogeneous pattern of defects in the 1,25(OHJzD, receptor-effector system of DDII patients. PBMC from these same patients exhibited identical defects in 1,25(OH),D, binding and in the biological response to the hormone as assessed by inhibition of mitogenesis (Karen et al., 1985). The mutations underlying the functional defects in the receptor were identified in cultured skin-derived fibroblasts or Epstein-Barr virustransformed lymphocytes from DDII patients. Various point mutations in the receptor gene were detected in the regions encoding the DNA binding domain and the hormone binding domain (Hughes et al., 1988; Malloy et al., 1990; Sone et al., 1990). Since the point mutations were detected in the receptor gene itself, it may be inferred that the same structural gene is coding for the 1,25(OH),D, receptor in PBMC, fibroblasts and presumably other vitamin D target organs. PBMC can thus serve as a valid target organ model for the identification of genetic defects in the 1,25(OH),D, receptor-effector system. It remains, however, to be established whether PBMC can also reflect acquired or transient modulation of 1,25(OH),D, target organ reactivity, for example during development and in various physiological and pathological states. In contrast with PBMC, which are assayed immediately upon isolation, skin fibroblasts undergo many divisions in culture before hormone responsiveness can be assayed. Therefore, the fibroblasts model system may not be expected to reflect transient or acquired changes in reactivity to 1,25(OH),D,. Evidence for the validity of the PBMC as a model for developmental changes in responsiveness to 1,25(OH),D, was obtained in a comparative study of human neonates and adults. Clinical evidence suggests that reduced responsiveness to 1,25(OH),D, occurs in the intestine and bone of very low-birth-weight infants. Delayed bone mineralization, with or without the clinical syndrome of rickets, is a common finding in these infants and may be due, at least in part, to inefficient intestinal calcium absorption (Shaw, 1976); this defect in calcium absorption exists despite normal or even elevated serum 1,25(OH)zD, levels (Steichen et al., 1981). Secondly, massive doses of 1,25(OH),D, were required before significant increases of osteocalcin were observed in such infants (Koo et al., 1986). In line with these findings we have shown that 1,25(OH),D, inhibited mitogen-induced stimulation of PBMC from newborns to a lesser extent than in adult donors (Ravid et al., 1988). The lower inhibitory action of 1,25(OH),D, in neonatal lymphocytes cannot be attributed to fewer receptors for the hormon