Monocyte heterogeneity in angiotensin-converting enzyme induction mediated by autologous T lymphocytes

The ability of monocyte subpopulations to be induced selectively by T lymphocytes to synthesize enhanced levels of angiotensin-converting enzyme (ACE) was examined using an in vitro model employing normal peripheral blood monocytes and T lymphocytes. Separation of monocytes into subpopulations on the basis of buoyant density indicated no difference in the ability of the resulting monocyte subpopulations to produce basal levels of ACE when cultured in the absence of T lymphocytes. However, the subpopulations differed significantly in their ability to synthesize enhanced levels of ACE in response to the presence of autologous T lymphocytes; low-density monocytes were induced by T lymphocytes to synthesize three-fold more ACE than were high-density monocytes. Surface antigen labelling using MoAbs demonstrated that the low-density monocyte subpopulations also had a significantly higher percentage of Leu-M2+ monocytes compared with the high-density monocyte subpopulations. When monocytes were separated on the basis of the presence of the Leu-M2 antigen using an immune rosetting technique, T lymphocytes were able to induce significantly elevated levels of ACE in the Leu-M2+ enriched monocyte subpopulation but were unable to induce ACE beyond basal levels in the Leu-M2(+)-depleted monocyte subpopulation. These results demonstrate that monocytes are heterogeneous with respect to their ability to be induced by T lymphocytes to synthesize ACE. This raises the possibility that selective accumulation of a monocyte subpopulation in the granulomatous inflammation of sarcoidosis may be one of the factors required for elevated ACE synthesis in the resulting granuloma epithelioid cells.

Comparison of the T lymphocyte-dependent induction of angiotensin-converting enzyme and leucine aminopeptidase in cultured human monocytes

The T lymphocyte-mediated induction of angiotensin-converting enzyme (ACE) in cultured autologous peripheral blood monocytes has been proposed as a model system for the investigation of the in vivo induction of ACE in the monocyte-derived granuloma epithelioid cells of some granulomatous diseases such as sarcoidosis. The studies described here were designed to evaluate the specificity of the model system by comparing the parameters for induction of ACE with those for the induction of another monocyte metallo-ecto-peptidase, leucine aminopeptidase (LAP). The concentration of LAP in freshly isolated monocytes was 0.09 mU/10(6) monocytes (s.e.m. 0.04) and increased to a maximal value of 0.19 mU/10(6) monocytes (s.e.m. 0.32) after 3 days when monocytes were cultured alone. ACE was not detectable in freshly isolated monocytes. However, after 6 days of culture, monocytes contained 0.22 mU ACE/10(6) monocytes (s.e.m. 0.04). Comparison of the levels of ACE and LAP induced during culture of monocytes alone indicated that the induction of these two enzymes were correlated. The induction of both enzymes was further enhanced by the presence of T lymphocytes in a dose-dependent manner. At 4 x 10(6) T lymphocytes per culture, ACE levels increased to 1.81 mU/10(6) monocytes (s.e.m. 0.24) and LAP levels to 1.03 mU/10(6) monocytes (s.e.m. 0.35). The enhancement of ACE activity required autologous lymphocytes, while heterologous T lymphocytes were equally effective in inducing LAP. Comparison of the levels of ACE and LAP induced during coculture of autologous T lymphocytes and monocytes from 21 independent donors, demonstrated no correlation between the induction of ACE and LAP. These data indicate that, although T lymphocytes also enhance the induction of LAP, the underlying mechanism must differ from that of ACE induction.