Fc receptors on monocytes cause OKT3-treated lymphocytes to internalize T3 and to secrete IL-2.

Monocytes cause OKT3-treated T cells to secrete IL-2 and to lose cell surface T3. We have studied the fate of the "lost" T3. Immunofluorescence microscopy on permeabilized cells showed that monocytes induce T cells to internalize T3. Furthermore, experiments with radioiodinated T cells showed that the internalized T3 was not degraded and exhibited an unaltered polypeptide composition for at least 16 hr. The role of Fc receptors in inducing internalization was also investigated. Fc receptors were depleted from monocytes by allowing the phagocytes to spread on immune complexes. Such depleted monocytes exhibit a fourfold reduction in their ability to promote both internalization of T3 and production of IL-2. A comparable reduction is seen if F(ab')2 fragments of OKT3 were employed in place of intact IgG. Furthermore, monocyte Fc receptors that have been blocked by heat-aggregated human IgG also show much reduced capability for induction of OKT3-mediated T-cell proliferation. We therefore conclude that Fc receptors bind to the Fc domain of OKT3 and thereby cause OKT3-treated T cells to internalize T3 and become activated.

Immunological studies of ageing. X. Impaired T lymphocytes and normal monocyte response from elderly humans to the mitogenic antibodies OKT3 and Leu 4.

Lymphocytes from old and young humans were cultured with PHA or the monoclonal antibodies OKT3 or Leu 4. The incorporation of [3H]TdR was significantly lower in cultures from old as compared to young donors, and the response of lymphocytes stimulated with OKT3 was the best discriminator of donor age. The mitogenic response of lymphocytes to these monoclonal antibodies requires monocytes. The response of T cells containing less than 5% adherent cells was diminished and the difference between old and young donors was not seen. The age-associated response was recovered when autologous or allogeneic monocytes were added to T cells. The age-associated response of T cells was the same, whether cultured with monocytes from young or old donors. Thus, monocytes from elderly subjects are not impaired with respect to their capacity to facilitate the proliferative response of T cells stimulated with monoclonal antibodies. Although lymphocytes from elderly donors were more sensitive to the inhibitory effect of prostaglandin E2, this did not account for the age-associated defect as indomethacin did not eliminate this defect. We conclude that the proliferative response of lymphocytes to OKT3 and Leu 4 is a more sensitive discriminator of lymphocyte donor age than is response to plant lectins, and that the age-associated defect in this response appears to reside within the T-cell population and not the monocyte population.

Modulation induction of the T3 antigen by OKT3 antibody is monocyte dependent.

We investigated the influence of monocytes on the susceptibility of the T3 antigen on human T cells to modulation induction by OKT3 antibody. In the absence of monocytes, the T3 antigen was only minimally susceptible to modulation. After the addition of 20% monocytes to the culture, however, complete modulation was readily observed. Furthermore, we found that even in the absence of OKT3 antibody, monocytes were able to down-regulate the expression of the T3 antigen, although to a lesser extent. The ability of monocytes to enhance antigenic modulation proved to be a more general phenomenon. Each individual T cell antigen, however, differed in its susceptibility to modulation by antibody, monocytes, or both, thereby establishing its own characteristic pattern. In addition, after complete modulation of the T3 antigen, the addition of monocytes to the culture thereafter had a distinct inhibitory effect on the reexpression of the T3 antigen. Monocyte enhancement of T3 modulation is significantly reduced when using the OKT3 F(ab')2 fragment, as is OKT3 mitogenesis. After pulsing the monocytes with OKT3 antibody before adding them to the culture, T3 modulation became nearly complete even in the absence of added OKT3 antibody. Monocyte-induced modulation proved not to be MHC restricted, thus allowing for comparative analysis of this effect between monocytes and other cell types. A moderate, however, incomplete modulation enhancement was observed with the human monocyte cell line U937 and with Daudi cells. This finding proved to coincide with the distinct ability of these cell lines to bind OKT3 antibody by their Fc receptors, as was the case with monocytes. In contrast, neither Fc receptor binding nor T3 modulation enhancement was observed with the cell lines Cess and G7. In addition, no effective T3 modulation was observed with glutaraldehyde-fixed monocytes. The overall results seem to indicate that effective modulation of the T3 antigen by OKT3 antibody requires the active participation of Fc receptors on monocytes.

OKT8 antibody inhibits OKT3-induced IL 2 production and proliferation in OKT8+ cells.

We studied IL 2 production and proliferation induced by OKT3 mitogenic monoclonal antibody in the OKT8+ T cell subset. OKT3 antibody induced IL 2 production and proliferation in OKT8+ cells in a typical time-dependent manner: maximal IL 2 levels were found in 24 hr culture supernatants; maximal proliferation was found on day 3. OKT3 antibody was mitogenic over a wide range of concentrations (0.125 to 500 ng/ml). The presence of OKT8 antibody (greater than or equal to 100 ng/ml) in these cultures resulted in almost complete inhibition of IL 2 production and proliferation. Kinetic studies demonstrate that OKT8 antibody suppresses both IL 2 production and response to exogenous IL 2 in OKT8+ cells when added within the first 2 hr of culture. After 14 to 20 hr of culture, addition of OKT8 only blocks IL 2 production but not the IL 2 response of activated OKT8+ cells. The specificity of inhibition by OKT8 antibody of OKT3 mitogenicity on OKT8+ cells was confirmed by the failure of Leu-I and OKT4 antibody to produce the same effect and by the lack of inhibition by OKT8 antibody of OKT3-induced IL 2 production and proliferation in OKT4+ cells.

Idiotype-like molecules on cells of a human T cell leukemia.

Two monoclonal antibodies were obtained that showed unique specificities for the leukemic T cells used for immunization. One antibody, S160, was totally specific for the antigen. The other antibody, S511, also reacted with a small population of normal T cells. This was made especially evident by concentrating these normal T cells with the antibody. Considerable evidence was obtained that both antibodies reacted with the same membrane molecules. In the unreduced state a major component of approximately 80 kdaltons was observed; after reduction this split into two components of approximately 43 and approximately 38 kdaltons. The reaction of the two antibodies with different antigenic sites on the same molecule, one representing a private site and the other a more cross-reactive site, strongly suggests an antibodylike molecule, but composed of polypeptide chains differing from immunoglobulins.