THERE IS A FUNDAMENTAL GAP IN OUR UNDERSTANDING OF THE MECHANISM(S) OF LONG-?TERM VIRAL PERSISTENCE IN THE MALE REPRODUCTIVE TRACT (MRT). EQUINE ARTERITIS VIRUS (EAV) ESTABLISHES LOCALIZED, LONG-?TERM PERSISTENT INFECTION (LTPI; &GT;1 YEAR TO LIFELONG) IN THE MAJORITY OF THE INFECTED STALLIONS DESPITE THE PRESENCE OF A SYSTEMIC IMMUNE RESPONSE. EAV PERSISTS PRIMARILY WITHIN THE AMPULLAE OF THE VAS DEFERENS NOTWITHSTANDING THE PRESENCE OF SIGNIFICANT INFLAMMATORY CELL INFILTRATES. THE MECHANISM OF THIS SUCCESSFUL EVASION STRATEGY REMAINS UNDEFINED. THE LONG-?TERM GOAL IS TO BETTER UNDERSTAND HOW EAV EVADES THE LOCAL HOST IMMUNE AND INFLAMMATORY RESPONSES IN THE GLANDULAR TISSUES OF THE MRT DURING LTPI. THEREFORE, THE EXPERIMENTS PROPOSED IN THIS STUDY ARE DESIGNED TO BETTER UNDERSTAND THE IMMUNE RESPONSE BY THE HOST (STALLION) AND IMMUNE EVASION MECHANISMS UTILIZED BY EAV TO PERSIST IN THE STALLION REPRODUCTIVE TRACT USING A GROUP OF EXPERIMENTALLY INFECTED STALLIONS.IT HAS BEEN DEMONSTRATED THAT HORSES THAT CARRY ONE ISOFORM OF EQUINE CXCL16 (CXCL16S [SUSCEPTIBLE FORM]) GENE HAVE A UNIQUE IMMUNE CELL SUBPOPULATION (I.E. CD3+T LYMPHOCYTE SUBPOPULATION) SUSCEPTIBLE TO EAV INFECTION AND ARE PREDISPOSED TO BECOMING LTPI CARRIERS. IN CONTRAST, STALLIONS THAT HAVE CD3+T CELLS THAT ARE NOT SUSCEPTIBLE TO EAV INFECTION CARRY THE CXCL16R ISOFORM (RESISTANT FORM OF THE GENE) AND ARE NOT INCLINED TO BECOME LTPI CARRIERS. IT IS KNOWN THAT CXCL16S/R HAS AN IMMUNE MODULATORY FUNCTION WHEN IT BINDS THE RECEPTOR PROTEIN KNOWN AS CXCR6 PRESENT IN NUMBER OF IMMUNE CELLS. THE INTERACTION BETWEEN CXCL16S/R AND CXCR6 UPREGULATE OR DOWNREGULATE A SERIES OF IMMUNE REGULATORY PATHWAYS IN THE IMMUNE SYSTEM (KNOWN AS CXCL16/CXCR6 AXIS). THUS, THE OVERALL OBJECTIVE OF THIS APPLICATION IS TO IDENTIFY HOW EAV USES THE CXCL16/CXCR6 AXIS TO ITS ADVANTAGE TO ESTABLISH AND MAINTAIN LTPI IN THE MRT. THE CENTRAL HYPOTHESIS IS THAT EAV USES CXCL16S (SUSCEPTIBLE) TO INFECT CELLS AND EXPLOITS THE CXCL16/CXCR6 CHEMOKINE AXIS TO INDUCE A UNIQUE IMMUNOLOGICAL MICROENVIRONMENT AT THE SITE OF VIRAL PERSISTENCE IN THE STALLION REPRODUCTIVE TRACT, WHICH LEADS TO CD8+T CELL EXHAUSTION. THIS HYPOTHESIS HAS BEEN FORMULATED ON THE BASIS OF PRELIMINARY DATA PRODUCED IN THE APPLICANT'S LABORATORY, WHICH SHOWS INCREASED EXPRESSION OF CXCL16S IN THE MRT DURING LTPI. THE RATIONALE FOR THE PROPOSED RESEARCH IS THAT ATTAINING A COMPREHENSIVE UNDERSTANDING OF EAV'S ABILITY TO "SUBVERT" OR "HIJACK" THE CXCL16/CXCR6 AXIS WILL ALLOW RESEARCHERS TO IDENTIFY NOVEL MECHANISM(S) OF IMMUNE EVASION; THIS UNDERSTANDING HAS THE POTENTIAL TO BE TRANSLATED INTO BETTER COMPREHENDING THE IMMUNOPATHOGENIC OF OTHER HUMAN AND ANIMAL VIRUSES THAT PERSIST IN THE MRT. THIS HYPOTHESIS WILL BE TESTED BY PURSUING THREE SPECIFIC AIMS: 1) DETERMINE THE ROLE OF CXCL16 AND CXCR6 PROTEINS IN EAV INFECTION OF CD3+ T LYMPHOCYTES; 2) DELINEATE THE NATURE OF THE IMMUNOLOGICAL MICROENVIRONMENT AND ITS ASSOCIATION WITH THE CXCL16/CXCR6 AXIS IN THE AMPULLAE DURING EAV LTPI AND 3) DEFINE THE IMMUNOLOGICAL NETWORK ASSOCIATED WITH T CELL EXHAUSTION IN INFILTRATING CD8+ T LYMPHOCYTES AND ITS LINK WITH THE CXCL16/CXCR6 AXIS DURING LTPI. THE PROPOSED STUDIES USE A MULTIDISCIPLINARY APPROACH, WHICH ENCOMPASSES VIROLOGY, MOLECULAR AND CELL BIOLOGY, REPRODUCTIVE IMMUNOLOGY/ IMMUNOPATHOLOGY AND GENOMICS, TO ESTABLISH THE ROLE OF THIS AXIS IN EAV LTPI. THE APPROACH, WHICH WILL INITIATE A PARADIGM SHIFT IN VIRAL IMMUNOLOGY, IS INNOVATIVE BECAUSE IT INVESTIGATES HOW VIRUSES "EXPLOIT" THE CXCL16/CXCR6 AXIS TO MAINTAIN LTPI IN THE MRT USING A WELL-?DESCRIBED NATURAL ANIMAL MODEL. THE PROPOSED RESEARCH IS SIGNIFICANT BECAUSE FINDINGS FROM THIS STUDY WILL IDENTIFY NEW TARGETS FOR THE DEVELOPMENT OF EFFECTIVE IMMUNOTHERAPIES AND DRUGS TO INDUCE VIRAL CLEARANCE FROM THE MRT, WHICH CAN BE APPLIED TO HUMAN AND OTHER ANIMAL VIRAL INFECTIONS.