After binding a sensitizing tumor antigen, human leukocytes undergo a series of changes that lead to a loss of their glass-adherent properties; a phenomenon called leukocyte adherence inhibition (LAI). After surgery or when patients have a large tumor burden, their test results become negative. This study shows that in vitro incubation of the leukocytes for 5 min with PGE2 converted to positive the negative test, in an immunologically specific manner. The effect was critically dose-dependent, too little or too much did not alter the result. The same effect was achieved with PGE2, PGI2, aminophylline or other drugs that raise intracellular nucleotides, including dibutyryl cyclic AMP and dibutyryl cyclic GMP. Dibutyryl cyclic AMP stimulated a stronger response and 100 times less was needed than of dibutyryl cyclic GMP. Prostaglandins did not mediate LAI since Indomethacin failed to inhibit a positive test. Nonetheless, arachidonate metabolites were critical for the LAI phenomenon since BPB and mepacrine, inhibitors of phospholipase A2, negated the LAI response. Moreover, ETYA, phenidone and NDGA, inhibitors of the lipoxygenase metabolic pathway, all negated the positive LAI response. The positive response was especially sensitive to nullification by ETYA. Although the last-named drugs inhibit other arachidonate metabolic pathways too, conclusive evidence that the metabolites of the lipoxygenase pathway, and leukotrienes in particular, mediate the LAI response was the fact that FPL 55712, a competitive antagonist of SRS, nullified a positive response at levels as low as 10(-13) M. The results imply that prostaglandins were able to modulate the expression of LAI by affecting intracellular nucleotides, but leukotrienes, it seems, were the metabolites that mediated leukocyte nonadherence after monocytes recognized and bound tumor antigen.
The tube LAI assay measures accurately antitumor immunity in patients with early cancer but fails to detect up to 75% of patients with advanced cancer due to excess circulating organ-specific neoantigen (OSN). Substances such as prostaglandin E2 (PGE2) or aminophylline, which increase intracellular nucleotides in leukocytes of patients with advanced cancer reversed this nonreactivity and greatly increased the sensitivity of the assay without any loss of specificity. Antitumor immunity can now be detected in advanced cancer, and a combination of the two assays gives prognostic potential to the assay: a positive test with PGE2 and negative test without indicates the patient has a large tumor burden. The specificity of the assay for each cancer was high and in most instances was greater than or equal to 95%. The PGE2 stimulated assay retained the high specificity. The sensitivity of the regular tube assay was often low, 33-56% because of the many advanced cancer patients tested, whereas the PGE2 stimulated assay showed almost a two-fold increase in sensitivity, 67-93%. The diagnostic value of the assay was estimated by calculating the predictive value for different prevalences of cancer. It was found that at low prevalences of cancer as found in the general population, the assay had a low diagnostic value since few patients with a positive test would have the cancer tested for. With prevalences of cancer of 5% or greater as might be found in a tertiary care clinical setting, the assay would seem to have diagnostic value since one half or more patients with a positive test would have the cancer tested for. Most false positives, but not all, are found in patients who have lesions that are often considered to increase their risk for cancer: severe dysplasia of the breast, colon adenomas, chronic atrophic gastritis and chronic pancreatitis, suggesting that the assay predicts oncogenesis.
Human peripheral blood monocytes were highly enriched by adherence to plastic, armed with serum from cancer patients, and challenged separately and simultaneously with the sensitizing and unrelated cancer extracts. The response of the monocytes was to release a factor that inhibited leukocyte adherence (LAI) to glass. The macrophage-like cell line U937 released a similar factor when it was armed and challenged with the sensitizing cancer extract. The production of the factor was blocked by 10(-6) M ETYA and 10(-6) M NDGA but not by 10(-6) M indomethacin. Moreover, a competitive inhibitor of leukotriene function, 10(-6) M FPL 55712, blocked the LAI reaction mediated by the factor. Arylsulfatase destroyed its activity while depletion of the monocytes' cellular glutathione pool with CyH or Et2Mal stopped production of the mediator. Pure leukotrienes (C and D) in a dose-response fashion prevented the adherence of leukocytes to glass; the nonadherence of mononuclear cells was equal to that of polymorphonuclear cells. PGE2, if added to the leukocytes immediately before challenge with LTC or LTD, increased 1,000-fold the leukocytes' sensitivity to the leukotrienes. Paradoxically, if leukocytes were washed and exposed to PGE2 15 min after being challenged with leukotrienes, their normal glass-adherence property and the ability to respond again to LTD were restored. FPL 55712 blocked the effect of LTC and LTD from inhibiting the adherence of leukocytes to glass. The present study shows that human monocytes armed with cytophilic anti-tumor antibody, when challenged with the sensitizing cancer extract, release leukotriene(s) as shown by pharmacologic evidence, implying that monocytes may play an important inflammatory role in human cancer.
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