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: The mechanisms of abnormal calcium metabolism in sarcoidosis need to be understood when treating hypercalcaemia, hypercalcuria and corticosteroid-induced osteoporosis. Studies are required to determine if the currently available therapies for osteoporosis are safe and effective in sarcoidosis.

SLMMARYThe deposition of circulating immune reactants in blood vessels, an important event in the pathogenesis of certain types of vasculitis, requires an increase in permeability in the endothelial monolayer. An in vitro model to examine the integrity of endothelial cell monolayers and their response to inflammatory mediators has been developed. Human umbiiieal vein endothelial cells were grown to confluence on an FITC-labellcd matrix and monolayer integrity was assessed by the exclusion of a '-M-anti-FITC antibody. Alteration in endothelial monolayer penneability was associated with an increase in uptake of '^^I-anti-FITC antibody, expressed as a percentage ofthe maximal uptake of antibody on to FITC-matdx frotn which endothelial cells had been stripped. We determined the effects on endothelial monolayer permeability of acute agonists (thrombin and histamine). cytokines (tumour necrosis factor-alpha (TNF-a). interferon-gamma (IFN-y), IL-I and IL-4) and combinations of acute agonists and cytokines. Addition of thrombin in concentrations ranging from 05 to 15 U/ml led to an increased uptake of'-'l-anti-FITC antibody from 2% to 15% relative to unstimulated endothelium. For other agonists and cyiokines the increases in permeability were: (i) histamine (50-400 pmo!/ml) increased uptake 5-22%: (ii) TNF (12 5-100 ng/ml) increased uptake 2-12%: (iii) IFN-7 (125-250 U/ml) inereased uptake I 5-3%. IL-l/i (50-100 U/ml) and U.-4 (50-100 U/ml) had no effect. Synergistic interactions on endothelial monolayer permeability were seen with the following combinations: (i) IL-4 (100 U/ml) and TNF (12-5 ng/ml) uptake 11 %; (ii) IL-4 (100 U/ml)and IFN-7 (125 U/ml) uptake6-5%; (iii)TNF(12-5ng/ml) and IFN-v (125 ng/ml) uptake 7%; (iv) thrombin (0 5 U/ml) and histamine (50 pmol/ml) uptake 13 5"/;,: and (v) TNF (12-5 ng/ml) and thrombin (0 5 U/ml) uptake 8 5'^. These observations suggest that interactions between cytokines and acute inflammatory mediators such as thrombin and histamine may be important in determining whether immune complexes are deposited in vessel walls. This model system may now be useful for the further investigation in vitro ofthe mechanisms involved in the pathogenesis of immune complex-mediated vascular damage.

Modification of the cellular immune response in uraemia is partly responsible for the increased susceptibility to infection found in dialysis patients. In order to study this further we have evaluated the in vitro production of tumour necrosis factor (TNF) by peripheral-blood monocytes (PBMCs) to stimulation by lipopolysaccharide (LPS) from dialysis patients with end-stage renal failure. The patients were subdivided into two groups according to the type of dialysis: those undergoing haemodialysis (HD; n ≈ 12) and continuous ambulatory peritoneal dialysis (CAPD; n = 18). Results were compared with those of controls taken from healthy laboratory staff (n = 7). The experiments show that the secretion of TNF by PBMCs in response to LPS is significantly augmented in patients undergoing HD when compared to those on CAPD (81.3 ± 38.7 vs. 18.2 ± 13.3 U/ml, mean ± SD, p < 0.001) and controls (81.3 ± 38.7 vs. 18.1 ± 6.6 U/ml, p < 0.001). There was no significant difference between the CAPD group and controls. In vitro production of TNF fell slightly following a single HD session (81.3 ± 38.7 U/ml before HD and 50.5 ± 28.7 U/ml after HD, p < 0.05). We conclude from this study that TNF release from PBMCs is augmented in patients with chronic renal failure receiving chronic HD but not in a similar group receiving CAPD, in vitro. TNF release, however, is suppressed immediately following a single HD session. We suggest that HD rather than uraemia per se up-regulates monocyte secretion of TNF in vitro and that this is not an immediate response to activation by membrane polymer.