Immune interventions consisting in repeated injections are broadly used as they are thought to improve the quantity and the quality of the immune response. However, they also raise several questions that remain unan-swered, in particular the number of injections to make or the delay to respect between different injections to achieve this goal. Practical and financial considerations add constraints to these questions, especially in the framework of human studies. We specifically focus here on the use of interleukin-7 (IL-7) injections in HIV-infected patients under antiretroviral treatment, but still unable to restore normal levels of CD4 + T lymphocytes. Clinical trials have already shown that repeated cycles of injections of IL-7 could help maintaining CD4 + T lymphocytes levels over the limit of 500 cells/µL, by affecting proliferation and survival of CD4 + T cells. We then aim at answering the question : how to maintain a patients level of CD4 + T lymphocytes by using a minimum number of injections (i.e., optimizing the strategy of injections) ? Based on mechanistic models that were previously developed for the dynamics of CD4+ T lymphocytes in this context, we model the process by a piecewise deter-ministic Markov model. We then address the question by using some recently 2 Chloé Pasin et al. established theory on impulse control problem in order to develop a numerical tool determining the optimal strategy. Results are obtained on a reduced model, as a proof of concept: the method allows to define an optimal strategy for a given patient. This method could be applied to optimize injections schedules in clinical trials.< Réduire