To obtain a preliminary indication as to whether adaptive immune responses could be stimulated by c-di-GMP, we immunized mice with human serum albumin (HSA) and tested whether coinjected c-di-GMP could function as an adjuvant, as previously reported (Ebensen et al., 2007; DAI, and at least one of these uncharacterized DNA sensors, or another independent sensor, may respond to c-di-GMP.