Methods used to evaluate the immune protection factor of a sunscreen: Advantages and disadvantages of different in vivo techniques

Solar UV radiation (UVR) induces skin cancer in humans, a process partially mediated by UVR-induced immunosuppression. To help prevent skin cancer, sunscreens should prevent UVR-induced immunosuppression to a level that is comparable to their sun protection factor (SPF). There are no standardized protocols for determining the immune protection factor (IPF) of sunscreens or agreement on what degree of IPF is needed to fully preserve cutaneous immune function. Current in vivo approaches to this problem rely on sunscreens' ability to prevent localized UVR-induced suppression of contact hypersensitivity (CHS) and delayed-type hypersensitivity (DTH) responses, using either the induction or elicitation arms of these responses, with comprehensive controls. The induction arm of the CHS response is sensitive to a single suberythemal exposure of solar-simulated radiation (SSR), and allows for direct SPF and IPF comparisons. However, this approach requires a large number of volunteers and is extremely labor-intensive and time-consuming. The elicitation arm of the CHS or DTH response exploits prior sensitization to contact or recall antigens. Fewer volunteers are needed, but current protocols require repeat SSR exposure, which may invalidate comparison with SPF measures based on a single exposure, or erythemal doses, which may have systemic effects. Robust protocols for routinely assessing IPF are needed.