Abstract: In some embodiments provided herein is a method of preparing cargo-loaded platelets, comprising: treating platelets with a cargo and with a loading buffer comprising a salt, a base, a loading agent, and optionally ethanol, to form the cargo-loaded platelets.

Abstract: In one aspect, provided herein are mutated influenza virus neuraminidase polypeptides, wherein the mutated influenza virus neuraminidase polypeptides comprise a first cytoplasmic domain, a first transmembrane domain, a first stalk domain, and a first globular head domain of a first neuraminidase of a first influenza virus with an insertion of 15 to 45 or 1 to 50 amino acid residues in the first stalk domain of the first neuraminidase. In another aspect, provided herein is an influenza virus comprising such a mutated influenza virus neuraminidase polypeptide, a genome comprising a nucleotide sequence encoding such a mutated influenza virus neuraminidase polypeptide or both. In another aspect, provided herein is an immunogenic composition comprising such an influenza virus, and optionally an adjuvant.