Cycic di-GMP, or a cyclic dinucleotide analogue thereof that has the same effect as cyclic di-GMP, stimulates or enhances immune or inflammatory response in a patient or enhances the immune response to a vaccine by serving as an adjuvant. Cyclic di-GMP, or a cyclic dinucleotide analogue thereof, also has neuroprotective properties for use as a neuroprotective agent to inhibit, treat, or ameliorate the effects of injuries, diseases,