Originally published by eLife, the original publication can be found at http://dx.doi.org/10.7554/eLife.03180 ; The co-stimulatory molecule CD28 is essential for activation of helper T cells. Despite this critical role, it is not known whether CD28 has functions in maintaining T cell responses following activation. To determine the role for CD28 after T cell priming, we generated a strain of mice where CD28 is removed from CD4+ T cells after priming. We show that continued CD28 expression is important for effector CD4+ T cells following infection; maintained CD28 is required for the expansion of T helper type 1 cells, and for the differentiation and maintenance of T follicular helper cells during viral infection. Persistent CD28 is also required for clearance of the bacterium Citrobacter rodentium from the gastrointestinal tract. Together, this study demonstrates that CD28 persistence is required for helper T cell polarization in response to infection, describing a novel function for CD28 that is distinct from its role in T cell priming. ; This work was supported by the Wellcome Trust [Project Grant, 083650/Z/07/Z], the Lister Institute of Preventive Medicine [Lister Prize Fellowship], the National Health and Medical Research Council [Career Development Award, 596868] and the Wellcome Trust [098051].