Molecular cloning of rat interleukin 4 cDNA and analysis of the cytokine repertoire of subsets of CD4+ T cells

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Abstract
Rat peripheral CD4+ T cells may be subdivided into two functionally distinct subpopulations (OX‐22highCD4+ and OX‐22lowCD4+) on the basis of their reactivity with the monoclonal antibody MRCOX‐22, which recognizes a restricted epitope on the leukocyte common antigen (LCA, CD45). Previous studies have demonstrated the increased activity of the OX‐22highCD4+ subset in assays of cell‐mediated reactivity, whereas the reciprocal OX‐22lowCD4+ subset provides the majority of help for B cells in secondary antibody responses. Analyses of in vivo function have subsequently shown that the autoreactive activity associated with the OX‐22highCD4+ subset can be inhibited through a suppressor activity within the OX‐22lowCD4+ subset, indicating a further immunoregulatory role for these cells. Since the differential production of lymphokines such as interferon‐γ (IFN‐γ) and interleukin 4 (IL4) is believed to regulate alternative effector responses to a particular antigen, we have compared the lymphokine mRNA profiles of activated OX‐22highCD4+ and OX‐22lowCD4+ subsets using nucleic acid probes specific for rat IL 2, IFN‐γ and IL4, the latter of which has been isolated by a polymerase chain reaction cloning technique and its sequence is described. A higher frequency of cells expressed IL 2 mRNA in the OX‐22high subset, in accordance with the relative levels of IL 2 protein produced. In contrast, more IFN‐γ mRNA was detected in the OX‐22lowCD4+ subset 24 h after mitogenic stimulation although these cells have consistently been shown to produce less IFN‐γ protein than the OX‐22highCD4+ subset. This apparent paradox was resolved by the finding that the IFN‐γ mRNA levels in the OX‐22lowCD4+ subset declined rapidly after 24 h while the levels continued to rise in the OX‐22highCD4+ population such that at 48 h the relative levels were reversed. We have also demonstrated a higher level of IL 4 mRNA expression within the OX‐22lowCD4+ subset, which is undoubtably involved in the increased B cell helper activity mediated by this subpopulation and may be responsible, in part, for their active suppression of cell‐mediated immune responses.