In 1976, interleukin-2 was discovered and purified which allowed researchers to grow T-cells and study their immunology, which led to the discovery of human T-cell leukemia virus (HTLV), the first retrovirus identified in humans.

IRP researchers led by Robert Gallo, M.D. (external link), found a way to successfully grow T-cells in culture for more than nine months. They accomplished the feat by first stimulating lymphocytes with phytohemagglutinin, a protein found in plants, and then examining for the production of potential growth factors in the culture fluid. The team identified T-cell growth factor (TCFG), now known as interleukin-2 (IL-2).