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High blood eosinophil counts in humans are usually due to parasitic infections, allergic processes or malignant diseases. Interleukin-5 (IL-5) is thought to be the principal eosinopoietic stimulus in most of these patients. As the causes of persistent eosinophilia in patients with the idiopathic hypereosinophilic syndrome (HES) are (by definition) unknown, a semi-quantitative assay for IL-5 mRNA in eosinophils and mononuclear cells was carried out using samples from 11 patients with HES. In three patients, unstimulated peripheral blood mononuclear cells (PBMC) and bone marrow mononuclear cells (BMMC) had markedly raised levels of IL-5 mRNA. In four of five patients studied, phorbol myristate acetate (PMA) stimulation of PBMC induced them to produce more IL-5 mRNA than four healthy subjects. In five patients, bone marrow IL-5 mRNA levels were related to blood eosinophil counts. Blood eosinophils from all six patients tested, and bone marrow granulocytes from four patients had undetectable levels of IL-5 mRNA. It was concluded that HES can be separated into two groups. One has high levels of IL-5 mRNA and/or an enhanced IL-5 mRNA response to stimulation. They may well respond to treatments which inhibit the effects of this cytokine on eosinophil progenitor cells. The second group appears to have a disease which is IL-5 independent.

As the carbohydrate lacto-N-fucopentaose III (CD15 antigen or X-determinant) and its sialylated derivative sialyl-Lewis X are involved in the adhesion of cells rolling along the surface of endothelial cells, experiments were done to study the presence of these molecules on human eosinophils from patients with the idiopathic hypereosinophilic syndrome. Normal-density eosinophils from some patients showed higher levels of expression for lacto-N-fucopentaose III than light-density eosinophils. In contrast, sialyl-Lewis X was highly expressed by light-density eosinophils. Activation of normal-density eosinophils with calcium ionophore A23187 resulted in increased expression of these molecules for a short time. Monoclonal antibodies to these carbohydrates stimulated eosinophils to secrete eosinophil cationic protein, but not eosinophil peroxidase, and acted as costimulatory signals for C3b-induced degranulation of eosinophil cationic protein. It was suggested that CD15 and sialyl-Lewis X might contribute to eosinophil-mediated tissue injury in patients with the idiopathic hypereosinophilic syndrome.

Experiments were done to see whether C3 or C3-split products are involved in lymphocyte recirculation, with particular reference to B lymphocytes which have C3b receptors. Rats were injected with cobra venom factor (CVF), and the output of subclasses of lymphocytes was measured in thoracic duct lymph in hourly collections during the subsequent 24 h. During the period of acute C3 activation which lasted for 2-8 h, the output of lymphocytes decreased by 47%, but returned to normal at later times, when C3 levels were reduced to less than 20% normal. There was no effect on the output of C3b receptor lymphocytes, and this receptor was not blocked probably because initial C3 levels in lymph were only 13% of blood levels, so that only small amounts of C3b were generated in lymph. When these lymphocytes were labelled and injected i.v. they migrated with the slow rate which is characteristic of normal B lymphocytes. The main effect of CVF was to reduce the output of T lymphocytes by 58% during the phase of acute C3 activation. When normal thoracic duct lymphocytes were labelled and injected, their rate of reappearance in thoracic duct lymph was only reduced during this phase. It was concluded that recirculation of lymphocytes is not C3 dependent, and that insufficient C3b is generated in lymphoid tissues to block C3b receptors on B lymphocytes during periods of rapid C3 activation. However the migratory rate of T lymphocytes through these tissues is reduced during this period, and it is suggested that this may be due to an effect of C3 split products on macrophages which lie along T-lymphocyte traffic routes.

Human vascular endothelial cells were isolated by collagenase digestion of umbilical veins. Hybridomas secreting monoclonal antibodies were raised by fusing a mouse myeloma cell line to spleen cells from mice immunized with the isolated endothelial cells. A clone was selected which produced an antibody binding strongly to human umbilical vein endothelial cells. This antibody, EN 3, was shown to be directed against a major antigen on the surface of the cells, and appeared to be distinct from other antigens previously described on vascular tissues. The antibody bound to a lesser extent to umbilical artery endothelial cells and syncytiotrophoblast. Capillary endothelial cells in adult oesophageal tissues and tonsil were also labelled by the antibody, as were capillaries in a seminoma and infiltrating duct carcinoma of the breast. This well defined distribution in some foetal, adult and malignant tissues suggests that there is structural heterogeneity amongst endothelial cells in different sites, which may be linked to differences in differentiation or function.

Colony formation by granulocyte/macrophage and eosinophilic progenitor cells in the blood and/or bone marrow of four patients with "hypereosinophilic syndrome' (HES) was studied in agar culture. Colony-stimulating activity (CSA) was derived from leucocyte feeder layers (LFL) or from medium conditioned by phytohaemagglutinin-stimulated lymphocytes (LCM). The total numbers of colonies in the patients' blood and marrow were normal. When patients' marrow cells were cultured over LFL, the proportion of colonies that were eosinophilic was greater than normal (mean 21% vs 6%, P less 0.001) and this difference was accentuated when CSA was derived from LCM (mean 53% vs 15%, P less than 0.001). LCM derived from normal subjects and LCM prepared from HES patients had similar effects. In the blood the total number of colonies and the proportion of eosinophilic colonies were similar in patients and controls. The overall pattern of colony formation HES differed distinctly from that observed in a patient with eosinophilic leukaemia reported previously. We conclude that the proportion of eosinophil-committed progenitor cells in the marrow of patients with HES is increased, but we have failed to demonstrate a role for the patient's own lymphocytes in augmenting eosinophil production.