** AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** PRRSV IS ONE OF THE MOST IMPORTANT SWINE VIRAL PATHOGENS. THE VIRUS MAINLY INFECTS PORCINE ALVEOLAR MACROPHAGES (PAMS). CD163 IS THE KEY CELLULAR RECEPTOR FOR THE VIRUS ENTRY INTO PAMS AND THE VIRAL MINOR GLYCOPROTEINS GP2 AND GP4 HAVE BEEN SHOWN TO INTERACT WITH CD163. IN VITRO, PRRSV IS MAINLY PROPAGATED IN MARC-145, THE MONKEY EPITHELIAL KIDNEY CELL LINE. CELLULAR FACTORS REQUIRED FOR PRODUCTIVE PRRSV INFECTION ARE NOT FULLY UNDERSTOOD. WE RECENTLY DISCOVERED A SINGLE AMINO ACID RESIDUE IN GP2 THAT DIFFERENTIATES PRRSV INFECTIVITY IN MARC-145 AND PAMS. PARTICULARLY, WILD-TYPE PRRSV CARRYING A LYSINE RESIDUE AT POSITION 160 (K160) IN GP2 REPLICATES EFFICIENTLY IN BOTH MARC-145 AND PAMS WHILEPRRSV MUTANTS CARRYING AN ISOLEUCINE RESIDUE AT POSITION 160 (I160) IN GP2 ONLY INFECTS MARC-145 CELLS BUT NOT PAMS. WE HYPOTHESIZE THAT THERE MIGHT BE UNIDENTIFIED CELLULAR FACTORS THAT CONTROL PRRSV REPLICATION IN THESE TWO DIFFERENT CELL TYPES. TO DEMONSTRATE THIS HYPOTHESIS, WE PROPOSE THREE OBJECTIVES. OBJECTIVES 1: COMPARATIVE ANALYSIS OF PRRSV ENTRY PATHWAYS INTO DIFFERENT CELL TYPES. OBJECTIVE 2: IDENTIFY ADDITIONAL CELLULAR FACTORS REQUIRED FOR PRRSV INFECTION. OBJECTIVE 3: DETERMINE THE POTENTIAL ALTERNATIVE CELLULAR TARGETS FOR PRRSV REPLICATION IN PIGS. KNOWLEDGE GAINED FROM THIS STUDY WILL BE CRITICAL FOR THE RATIONALE DEVELOPMENT OF SAFE AND EFFICACIOUS PRRSV VACCINE. ADDITIONALLY, THIS KNOWLEDGE WILL PROVIDE NOVEL INSIGHTS FOR THE GENERATION OF PRRSV RESISTANT PIGS, EITHER THOUGHT NATURAL SELECTION OR GENOME-EDITING.