Influenza A virus hemagglutinin (HA) initiates viral entry by engaging host receptor sialylated glycans via its receptor-binding site (RBS). The amino acid sequence of the RBS naturally varies across avian and human influenza virus subtypes and is also evolvable. However, functional sequence diversity in the RBS has not been fully explored. Here, we performed a large-scale mutational analysis of the RBS of A/WSN/33 (H1N1) and A/Hong Kong/1/1968 (H3N2) HAs. Many replication-competent mutants not yet observed in nature were identified, including some that could escape from an RBS-targeted broadly neutralizing antibody. This functional sequence diversity is made possible by pervasive epistasis in the RBS 220-loop and can be buffered by avidity in viral receptor binding. Overall, our study reveals that the HA RBS can accommodate a much greater range of sequence diversity than previously thought, which has significant implications for the complex evolutionary interrelationships between receptor specificity and immune escape.

甲型流感病毒血凝素（HA）通过其受体结合位点（RBS）与宿主受体唾液酸化聚糖结合，从而开始病毒进入。RBS的氨基酸序列在禽流感和人类流感病毒亚型中自然变化，并且也是可进化的。但是，RBS中的功能序列多样性尚未得到充分研究。在这里，我们对A / WSN / 33（H1N1）和A / Hong Kong / 1/1968（H3N2）HA的RBS进​​行了大规模的突变分析。鉴定出许多尚未在自然界中观察到的具有复制能力的突变体，包括一些可以从靶向RBS的广泛中和抗体中逃脱的突变体。RBS 220环中的普遍上位性使这种功能序列的多样性成为可能，并且可以通过病毒受体结合的亲和力来缓冲。全面的，