Evidence that serum NTx (collagen-type I N-telopeptides) can act as an immunochemical marker of bone resorption.

Previous studies have shown that immunoassay of urinary NTx (cross-linked N-telopeptides of type I collagen) provides a responsive index of human bone resorption. Here we report by a sensitive immunoassay that NTx is present in serum and is suppressed appropriately in patients with Paget disease of bone by bisphosphonate antiresorptive therapy. The monoclonal antibody (1H11) developed against urinary NTx was applied in a sensitive chemiluminescence format. Results for human serum and urine showed parallel inhibition curves. The NTx concentrations in paired serum and urine samples from individual patients correlated well when urinary concentrations were normalized to creatinine concentrations (in premenopausal and postmenopausal women and Paget disease patients, r = 0.90, n = 60). The percentage of NTx suppression from baseline values for Paget disease patients on bisphosphonate therapy was similar for serum and urine. Blood samples drawn from bone marrow at the site of Pagetic lesions in three patients with active disease had as much as 10-fold higher concentrations of NTx than did peripheral blood samples drawn at the same time. The latter finding is consistent with other evidence showing that immunoreactive NTx originates directly during the proteolytic cleavage of bone collagen by osteoclasts rather than, e.g., by degradative processes occurring later in the liver and kidney.

Kinetic analysis of antibody-dependent cellular cytotoxicity: evidence for noncompetitive inhibition by autologous lymphoid cells.

Inhibition of antibody-dependent cellular cytotoxicity (ADCC) by autologous lymphocytes was analyzed by using classical techniques for enzyme-substrate interactions. We determined empirically that the interaction of murine spleen cells with antibody-coated targets to produce lysis was analogous to the interactions that have been described for an enzyme with its substrate. Varying numbers of antibody-coated target cells ("substrate") were mixed with a constant number of spleen cells ("enzyme") and the number of target cells killed ("product") was measured as a function of time. By analogy with Michaelis-Menten enzyme kinetics, two parameters of the reaction were determined: Vmax, the maximum velocity of lysis that is proportional to the number of killer cells present, and K1/2, an intrinsic property of the killer cells. These parameters were found to be independent variables. Addition of autologous lymph node cells produced a dose-dependent decrease in Vmax whereas K1/2 was not significantly changed. By analogy with enzyme kinetics, this inhibition is noncompetitive, suggesting that the autologous lymphocytes inactivate the killer cells rather than competing for the cell-cell binding sites.

Inhibition of antibody-dependent cellular cytotoxicity by autologous lymph node cells.

A population of lymph node cells that lack the usual T, B, or K cell markers was found to inhibit autologous spleen cells from mediating antibody-dependent cellular cytotoxicity (ADCC) to antibody-coated chicken erythrocytes. Inhibitor cells were not susceptible to treatment with anti-Thy 1.2 or anti-Ig and C; they did not adhere to Sephadex G-10, to nylon wool, or to monolayers of sheep erythrocytes (E) or erythrocytes plus 7S antibody (EA). After a brief (4-min) exposure to 45 degrees C, the ability to inhibit was lost whereas other cellular responses remained intact. ADCC mediated by nonadherent splenic effector cells (presumptive K cells) was highly susceptible to inhibition. Possible mechanisms for and implications of lymphocyte-mediated inhibition of ADCC are discussed.