Interleukin 2 pathway is autonomously activated in human T11⁺3⁻4⁻6⁻8⁻ thymocytes

Mitogenic membrane ligands have been shown to activate interleukin 2 (IL2) production only in mature T cells, IL2 constitutive secretion having not yet been demonstrated. Here we have isolated a population of T11⁺3⁻4⁻6⁻8⁻ human thymocytes and CD7⁺/T11⁻3⁻4⁻6⁻8⁻ prothymocytes which produce and consume IL2 upon phytohemagglutinin triggering. Interestingly, their proliferation in the absence of any exogenous stimulating agent was related to an autonomous use of the IL2 system (IL2 secretion and binding to its specific receptor, whose constitutive, functional expression in human early thymocytes was recently shown). The internal activation of this system before T cell receptor acquisition stresses the relevance of the understanding of the alternative activation pathway(s) in T cell development. These findings, together with the demonstration of IL2-promoted differentiation of human early thymocytes into mature T cells, suggest that IL2 may also be a growth and differentiation factor acting specifically early in T cell development.