Brief exposure of skin to near-infrared (NIR) laser light has been shown to augment the immune response to intradermal vaccination and thus act as an immunologic adjuvant. Although evidence indicates that the NIR laser adjuvant has capacity to activate innate subsets including dendritic cells (DCs) in skin as conventional adjuvants do, the precise immunological mechanism by which the NIR laser adjuvant acts is largely unknown. Here we sought to identify the cellular target of the NIR laser adjuvant by using an established mouse model of intradermal influenza vaccination and examining the alteration of responses resulting from genetic ablation of specific DC populations. We found that a continuous wave (CW) NIR laser adjuvant broadly modulates migratory DC populations, specifically increasing and activating the Lang+ and CD11b−Lang− subsets in skin, and that the antibody responses augmented by the CW NIR laser are dependent on DC subsets expressing CCR2 and Langerin. In comparison, a pulsed wave (PW) NIR laser adjuvant showed limited effects on the migratory DC subsets. Our vaccination study demonstrated that the efficacy of CW NIR laser is significantly better than that of PW laser, indicating that the CW NIR laser offers a desirable immunostimulatory microenvironment for migratory DCs. These results demonstrate the unique ability of the NIR laser adjuvant to selectively target specific migratory DC populations in skin depending on its parameters, and highlight the importance of optimization of laser parameters for desirable immune protection induced by a NIR laser-adjuvanted vaccine.

The treatment of skin with low-power continuous wave (CW) near-infrared (NIR) laser prior to vaccination is an emerging strategy to augment the immune response to intradermal vaccine, potentially substituting for chemical adjuvant, which has been linked to adverse effects of vaccines. This approach proved to be low-cost, simple, small, and readily translatable compared to the previously explored pulsed wave (PW) medical lasers. However, little is known on the mode of laser-tissue interaction eliciting the adjuvant effect. Here we sought to identify the pathways leading to the immunological events by examining the alteration of responses resulting from genetic ablation of innate subsets including mast cells (MCs) and specific dendritic cell (DC) populations in an established model of intradermal vaccination, and analyzing functional changes of skin microcirculation upon the CW NIR laser treatment in mice. We found that CW NIR laser transiently stimulates MCs via generation of reactive oxygen species (ROS), establishes an immunostimulatory milieu in the exposed tissue, and provides migration cues for dermal CD103+ DCs without inducing prolonged inflammation, ultimately augmenting the adaptive immune response. These results indicate that use of NIR laser with distinct wavelength and power is a safe and effective tool to reproducibly modulate innate programs in skin. These mechanistic findings would accelerate the clinical translation of this technology and warrant further explorations into the broader application of NIR laser to the treatment of immune-related skin diseases.