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IL-6/BSF-2 selectively stimulates the GO----S progression of CD8+ lymphocytes
D Bulgarini1, S Scalzo, G Boccoli
1Department of Hematology and Oncology, Istituto Superiore di Sanità, Roma, Italy.
Insights
Interleukin-6 (IL-6) selectively enhances DNA synthesis in CD8+ lymphocytes, not CD4+, via the CD2 pathway. This immune stimulation promotes cell cycle progression but not mitosis, offering insights into T-cell activation.
Area of Science:
- Immunology
- Cell Biology
Background:
- Interleukin-6 (IL-6) is a cytokine with diverse roles in immune responses.
- The specific mechanisms by which IL-6 influences T-lymphocyte activation, particularly CD8+ T cells, require further elucidation.
Purpose of the Study:
- To investigate the selective effects of IL-6 on DNA synthesis and activation pathways in human peripheral blood CD8+ and CD4+ lymphocytes.
- To determine the role of IL-6 in T-cell activation independent of IL-2 and accessory cells.
Main Methods:
- Purification of CD8+ lymphocytes (>99%) and culture in serum-free conditions.
- Stimulation with phytohemagglutinin (PHA), anti-CD2 monoclonal antibody (mAb), and anti-CD3 mAb.
- Assessment of DNA synthesis via (3H)-thymidine incorporation.
- Limiting dilution analysis to assess accessory cell requirement.
- Analysis of IL-6 receptor (IL6R) expression and binding kinetics (Scatchard analysis).
Main Results:
- IL-6 preferentially promoted DNA synthesis in CD8+ lymphocytes over CD4+ lymphocytes in the presence of PHA.
- IL-6 stimulated CD8+ lymphocyte DNA synthesis via anti-CD2 mAb but not anti-CD3 mAb, suggesting selective pathway activation.
- Accessory cells were not required for IL-6's action on CD8+ cells, and the effect was independent of IL-2.
- IL-6 induced G0 to S phase progression in CD8+ lymphocytes but did not lead to mitosis.
- Both resting and activated CD4+ and CD8+ lymphocytes expressed high and low affinity IL-6 receptors.
Conclusions:
- IL-6 directly and selectively stimulates the G0 to S phase progression of CD8+ lymphocytes in the presence of mitogens and absence of IL-2.
- The findings suggest IL-6 activates CD8+ lymphocytes through the CD2 pathway, distinct from the CD3 pathway.
- This selective action of IL-6 on CD8+ lymphocytes may be significant for understanding cytotoxic T lymphocyte activation mechanisms.
Abstract:
IL-6 preferentially promotes the DNA synthesis of human peripheral blood CD8+, rather than CD4+, lymphocytes in presence of PHA: this effect is observed in serum-free cultures of greater than 99% purified CD8+ lymphocytes. However, IL-6 is able to stimulate DNA synthesis of CD8+ lymphocytes triggered by a mitogenic anti-CD2 mAb, but not by anti-CD3 mAb: these results suggest that IL-6 selectively induces activation of CD8+ lymphocytes through the CD2 rather than the CD3 pathway. Limiting dilution analysis indicates that accessory cells are not required to mediate the action of IL-6 on CD8+ cells. Furthermore, this action is not blocked by addition of mAb neutralizing either IL-2 or IL2R, thus suggesting that IL-6 does not act via IL-2. CD8+ lymphocytes grown in the presence of PHA + IL-6 incorporate (3H)-thymidine to the same extent as those stimulated with PHA + IL-2, but do not increase in number until day 6 of culture. It is hence apparent that the stimulating activity of IL-6 on CD8+ lymphocytes is restricted to the GO----S phase progression, but does not lead to mitosis. IL-6 receptors are expressed on resting CD4+ and CD8+ lymphocytes: their expression is significantly enhanced on both activated CD4+ and CD8+ cells. Scatchard analysis of (125I)-IL-6 binding data showed the presence of high (Kd, 3 x 10(-10) M) and low (Kd, 6 x 10(-8) M) affinity IL6R on both lymphocyte populations. Similarly, mRNA encoding IL6R was detected in both CD4+ and CD8+ lymphocytes. Thus, our studies indicate that IL-6 directly and selectively stimulates the GO----S progression of CD8+ lymphocytes in the presence of mitogen and absence of IL-2: this phenomenon may be of interest for the elucidation of mechanisms activating cytotoxic T lymphocytes.