Interleukin 2 and low molecular weight B cell growth factor are T cell-replacing factors for different subpopulations of human B cells

Cytokines and T cell switching

In recent years, the phenotypic characterization of T cell subsets has given way to a functional dichotomy based essentially on their cytokine profiles. In this context, the CD4+ helper T cell subset has been shown to consist of two types, termed Th1 and Th2. In general, Th1 cells produce interleukin (IL)-2 and interferon (IFN)-gamma, while Th2 cells characteristically produce IL-4, IL-5, and IL-6. The major function of the Th1 subset is to mediate delayed-type hypersensitivity reactions and their secondary function is suppression of B cell activity. In contrast, the major function of the Th2 subset is to provide B cell help, while their secondary function is cell-mediated immune suppression. A similar dichotomy has also been described for CD8+ T cells. The role that these functional T cell subsets and their cytokines play in terms of their protective and nonprotective outcomes in a variety of infectious and oral diseases is reviewed.

Human T cell-replacing factor(s): a comparison of recombinant and purified human B cell growth and differentiation factors

Conditioned medium from phytohemagglutinin-activated T cells contains T cell-replacing factor(s) (TRF) able to restore specific antibody responses by human blood or tonsillar B cells which have been thoroughly depleted of T cells. Of twelve recombinant cytokines tested as possible candidates for TRF in conditioned media, namely human recombinant interleukin (hrIL) 1 alpha and beta, hrIL2, hrIL3, hrIL4, hrIL5, hrIL6, hrIFN-alpha and -gamma, hr granulocyte macrophage colony-stimulating factor (hrGM-CSF) and tumor necrosis factor (hr TNF)-alpha and -beta only IL2 was found to have TRF activity. In addition, a semi-purified low molecular weight B cell growth factor (BCGFlow) also had TRF activity. As the commercially available BCGFlow is known to contain low concentrations of IL2, IFN-gamma, TNF and GM-CSF as impurities, it was important to exclude these as being responsible for the TRF activity. At the concentrations present in BCGFlow (less than 0.2 U/ml), IL2 was not active in the TRF assay. In contrast, a combination of IL2 (0.2 U/ml), IFN-gamma (50 U/ml), TNF-alpha (50 U/ml) and TNF-beta (100 U/ml) did have TRF activity suggesting that B cells could be made to respond to low doses of IL2 by the presence of other cytokines. Although this finding raises important questions about the nature of TRF in conditioned medium, the TRF activity of BCGFlow was unlikely to be due to such a synergistic combination of cytokines for the following reasons. First, in several experiments, responses were obtained with BCGFlow, but not with IL2 or combinations of IL2 with IFN and TNF. Second, antibody to IL2 was found to inhibit the TRF activity of IL2 but not of BCGFlow. Taken together these findings show that two distinct cytokines (IL2 and BCGFlow) are TRF for human B cells. However, some combinations of cytokines can also have TRF activity underlining the complexities which can arise from working with semi-purified rather than recombinant factors.