Ultrastructural localization of retinal S antigen in the human retina

Retinal S antigen, a uveitogenic protein implicated in several forms of clinical and experimental uveoretinitis, has been localized by immunoelectron microscopy in the human photoreceptor outer segment using a protein A-gold technique and anti-bovine retinal S antigen serum. The protein was predominantly located to the proximal portion of the rod outer segment and appeared to be distributed on the disc membrane but not on the plasma membrane. No immunoreactive product was detected in cone outer segments or in any other retinal structure. These findings are discussed in relation to the physiological function of S antigen and its role in initiation of posterior uveitis.