The present invention relates to the field of adjuvant and vaccination. In the present study, the inventors investigate whether P1, in addition to being an antigen, could act as an adjuvant by first exploring its capacity to stimulate epithelial TSLP production. They evaluated additional immunomodulatory effects of P1 on human nasal mucosal models, including cytokines and chemokines production, intracellular signaling pathways, mucosal DC activation, T cell proliferation, and antigen-specific B cell responses against a model antigen in vitro. Altogether, they reported the immunological mechanism underlying P1-vaccine and the interest of P1 as a nasal mucosal adjuvant. Thus, the present invention relates to an immunoadjuvant composition comprising the P1 peptide of the HIV-1 envelope subunit gp41.