Mycosis fungoides (MF) and its leukemic variant, the Sezary syndrome (SS), are the most common forms of cutaneous T-cell lymphoma. Epidemiological studies indicate that the incidence of MF/SS is increasing, both in absolute terms and as a proportion of all lymphomas. Statistics also indicate that the incidence of HTLV-I retrovirus infection is increasing in the U.S.A. Following reverse transcription into proviral DNA, HTLV-I becomes integrated into the genome of host T-cells. After a latent period of 20-30 years, another form of cutaneous T-cell lymphoma known as adult T- cell leukemia/lymphoma (ATL) develops in 1% of infected individuals. ATL cases have been reported in the U.S.A., especially in the Southeast and Hawaii. Although MF/SS patients are characteristically seronegative for HTLV-I using conventional assays, recent ultrastructural, molecular biologic and serologic findings suggest that this retrovirus or a variant may play a role in MF/SS pathogenesis. Our unpublished data support this view. We have screened biopsies from 38-58 MF patients from different geographical regions for HTLV-I proviral DNA using a PCR-based assay for the HTLV-I pX, pol and env genes and have confirmed our findings using Southern blot analysis. Eleven of these cases were HTLV-I+ including six pX+polenv cases, two pX+pol+env cases, two pXpol+env cases and one pXpol+env case. The eleven HTLV-I+ cases included 4/5 California cases and 1/1 Chile case. In contrast only 6/29 Ohio area cases, 0/15 Swiss cases, 0/5 Massachusetts cases and none of other geographically scattered cases were HTLV-I+. Furthermore, 10-17 non-MF/SS skin disease controls from Ohio were negative for corresponding HTLV-I PCR products. PCR products have now been cloned and sequenced from the mT4 positive control cell line, from a pX+pol+ MF patient and from an env+ MF patient. Comparison to a cosmopolitan strain of HTLV-I from Japan indicates identical pX and pol sequences but unique env sequences. These results are the most extensive and definitive of their kind to date. The y provide very compelling support for the hypothesis that HTLV-I or a related retrovirus is involved in the pathogenesis of at least some cases of MF/SS and suggest a potential geographical clustering. This hypothesis is supported further by our highly sensitive analysis of MF/SS patient sera using Western blots spiked with recombinant HTLV proteins. Overall, 10/23 MF/SS patients have shown some evidence of HTLV seroreactivity, including 7/13 pacific Rim cases versus only 1/8 Ohio cases. Our objective in this research proposal is to confirm and extend our findings by using PCR/Southern blot analysis to test for additional HTLV-I genes in a larger number and geographical diversity of MF/SS patients. We will also use DNA cloning and sequencing techniques to determine if there are any common denominators among different HTLV-I= MF/SS cases, either in terms of proviral gene structure or site of proviral integration. In addition to helping clarify the pathogenesis of MF/SS and providing new approaches to diagnosis, prevention and therapy, this research could have broader implications for understanding the pathogenesis of other types of lymphomas that are associated with MF/SS.