In vitro studies on human IgD I. Sources and characteristics of “externalized” IgD in tonsil lymphocyte cultures

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Abstract
As part of a broader analysis into the function of IgD, and especially into the role of human IgD‐secreting cells, fresh human tonsil lymphocyte cultures were analyzed. The goals were to define the origins of “externalized” supernatant IgD and to examine its relationship to spontaneous Ig secretion of IgG, IgM and IgA. Assayable IgD was found by 24 h in cultures without added mitogens. “Externalized” IgD comprised a small fraction (1.6–1.8%) of supernatant Ig when compared to the fraction of IgD‐containing cells (≈ 10%): IgD appears to disappear rapidly in vitro. Initial experiments employing irradiation and chemical inhibitors of protein synthesis suggested that “externalized” IgD was produced de novo during culture rather than representing preformed or cytophilic Ig. Most significantly, tonsil cultures with higher IgD values (> 20 ng IgD/106 cells, day 7) showed convincing evidence that secreted IgD was a major source for “externalized” IgD. This evidence included (a) the amount of “externalized” IgD depended on viable culture conditions; (b) radioincorporated 35S appeared as a visible IgD band on sodium dodecyl sulfate‐polyacrylamidegel electrophoresis (SDS‐PAGE) gels; (c) “higher” IgD cultures showed increased numbers of IgD‐Ig‐containing plasma cells, and IgD,λ in excess of IgD, x; both events are associated with IgD secretion. In cultures producing high amounts of IgD, IgD secretion appeared to be spontaneous rather than induced and was independent of the polyclonal spontaneous Ig secretion of other Ig isotypes. Low titers of inhibitable IgD isotype anti‐phosphorylcholine antibodies but not IgD isotype anti‐polyribotol phosphate antibodies were present in tonsil supernatants: both were present in matched sera. Other laboratories have directed attention to the striking and possibly selective participation of human tonsil lymphocytes in IgD synthesis using other approaches. Present results supplement and expand earlier data and support the practical value of analysis of short‐term cultured tonsil lymphocytes. Differentiation to IgD‐secreting cells is suggested to be an active, underestimated and putatively an important part of the immune response in the human tonsil.