The identification of small molecules that affect T cell activation is an important area of research. Three molecules that regulate plant growth and differentiation, but not their structurally similar analogs, were identified to enhance primary mouse CD4+ T cell activation in conjunction with soluble anti-CD3 stimulation: Indoleacetic acid (natural plant auxin), 1-Napthaleneacetic acid (synthetic plant auxin) and 2,4-Dichlorophenoxyacetic acid (synthetic plant auxin and herbicide). These effects are distinct in comparison to Curcumin, the well known phenolic immunomodulator, which lowers T cell activation. An investigation into the mechanisms of action of the three plant growth regulators revealed a rapid induction of reactive oxygen species (ROS), mainly comprising H2O2. In addition, these three molecules synergize with soluble anti-CD3 signaling to enhance intracellular Ca2+ concentrations [Ca2+]i, leading to greater T cell activation, e.g. induction of CD25 and IL-2. Enhanced production of TNFα and IFNΓ by CD4+ T cells is also observed upon plant growth regulator treatment with soluble anti-CD3. Interestingly, maximal IL-2 production and CD4+ T cell cycle progression are observed upon activation with soluble anti-CD3 and phorbol 12-myristate 13-acetate (PMA), a phorbol ester. Additionally, stimulation with PMA and Ionomcyin (a Ca2+ ionophore), which activates T cells by circumventing the TCR, and plant growth regulators also demonstrated the role of the strength of signal (SOS): T cell cycle progression is enhanced with gentle activation conditions but decreased with strong activation conditions. This study demonstrates the direct effects of three plant growth regulators on CD4+ T cell activation and cycling.
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We are grateful to Prof. T. Ramasarma for introducing us to the fascinating world of intracellular free radicals and their biological effects. We thank Prof. C. Jayabaskaran for access to different plant growth modulators during the initial screening experiments. In addition, we appreciate the discussions with Dr. U. Nath on the effects of auxins on immune cells. The support and suggestions by members of the DpN laboratory during this study has been most useful. The expertise of the members of the FACS facility, Dr. O. Joy, Dr. W. Surin, M. Vamsi and P. Pai, is highly appreciated. This study was supported by a grant from the Department of Biotechnology, Government of India .
- Intracellular Ca
- Reactive oxygen species
- T cell activation