The major porin proteins present in the outer membrane of Neisseria meningitidis, the causative agent of life-threatening meningitis and septicaemia, are believed to have potent immunostimulatory effects. In this study, the interactions between human monocyte-derived dendritic cells (mo-DC) and the PorA porin were investigated, in order to reveal the role of this protein in promoting innate and adaptive immune responses. Recombinant (r)PorA induced mo-DC maturation, as reflected by reduced receptor-mediated endocytosis, increased production of the chemokines IL-8, RANTES, MIP-1α and MIP-1β and augmented expression of the surface markers CD40, CD54, CD80, CD86 and major histocompatibility complex class II molecules. However, rPorA induced either low level or no significant secretion of pro-inflammatory cytokines from mo-DC. The protein potently augmented the capacity of mo-DC to activate both allogeneic CD4+ memory T-cells and CD4+RA+ naïve T-cells. In addition, rPorA appeared to inhibit the production of IL-12p70 that follows from the interaction between CD40 on the mo-DC and CD40-ligand on T-cells, thereby directing T-cell differentiation towards a Th2 type response. These data demonstrate that PorA is involved in DC activation and in influencing the nature of the T-helper immune response, which are important properties for generating antibody responses required for protective immunity against meningococci and for determining the immuno-adjuvant effects of this protein.