The theory that the presence of 'helper' CD4⁺ T cells is absolutely necessary to stimulate CD8⁺ precursors into effector T cells is challenged by two papers in the May Nature Immunology. Marianne van Stipdonk and colleagues from La Jolla Institute for Allergy and Immunology, San Diego and Susan Kaech and colleagues from Emory University, Atlanta report that a brief antigen encounter might be enough to trigger transformation of naïve T precursors into efficient cytotoxic cells — even in the absence of helper T cells.

Marianne van Stipdonk et al used naïve TCR-transgenic CD8⁺ cells and an engineered antigen-presenting cell (APC) to study the requirements for priming and clonal expansion of naïve cytotoxic T lymphocytes. They found that naïve cytotoxic T lymphocytes become committed after as little as two hours exposure to APC, and that subsequent division and differentiation can occur without the need for...