Increase of immune response to viral antigens by a specific dsRNA motif. (a) The induction of antigen-specific IgG response in C3H/HeJ mice deficient in TLR-4 was measured by ELISA after immunization with OVA together with pI:pC (filled diamonds) or pA:pU (filled squares), or with OVA alone (open triangles). Results are expressed as mean ± SEM of OD (measured at 405 nm) corresponding to different serum dilutions (n = 3 mice/group) and are representative of two independent measurements. (b) The effect on the antibody response to the HIV gp140 fragment was measured in female BALB/c mice immunized via the respiratory tract (intratracheal administration followed by two boosts 2 weeks apart, carried out by intranasal instillation) with gp140 alone or gp140 together with pA:pU. The results were expressed as mean ± SEM of IgG endpoint titers (n = 3 mice/group). (c) Similarly, influenza virus–specific IgG antibodies were measured after mucosal immunization of female BALB/c mice with UV-inactivated WSN virus (UV-WSN) alone or together with dsRNA motifs. As control we measured the antibody response after infection with the same strain of influenza virus (n = 4/group). (d) The T cell response to whole influenza virus was studied by ELISpot analysis of splenocytes and was expressed as IFN-γ⁺ (gray bars), IL-4⁺ (white bars), and IL-2⁺ (black bars) SFCs/spleen (mean ± SEM, n = 4/group) after subtraction of background. The T cell response to antigen together with pA:pU was compared with the response to immunization with antigen alone or to influenza virus infection. The experiment was carried out in female BALB/c mice.