The 2022 North American outbreak of 2.3.4.4b H5N1 avian influenza virus revealed substantial mammalian adaptation and pathogenicity, yet mechanisms remain unclear. To address this knowledge gap, we investigated the North American H5N1 strain (GA/W22-145E/22), which demonstrated unique immune cell-mediated systemic dissemination, neuroinvasion, and 100% mortality in ferrets, unlike the nonlethal Eurasian strain (KR/W811/21). Genomic and reverse genetics studies identified PB2478I and NP450N mutations as key determinants of enhanced polymerase activity, immune cell tropism, and pathogenicity. Mutant GA/W22-145E/22 virus carrying PB2478V/NP450S showed complete survival without systemic dissemination. Furthermore, GA/W22-145E/22 demonstrated robust replication in human peripheral blood mononuclear cells and bovine mammary gland organoids, raising concerns about zoonotic spillover. These findings underscore PB2478I and NP450N as pivotal markers of pathogenicity, emphasizing the urgent need for enhanced surveillance and targeted interventions.