Modified Red Blood Cells and Uses Thereof for Delivering Agents

A method of increasing the circulation time or plasma half-life of an agent in a subject, comprising providing a sortase substrate that comprises a sortase recognition motif and an agent, and conjugating the sortase substrate in the presence of a sortase under conditions suitable for the sortase to conjugate the sortase substrate to at least one membrane protein of a red blood cell by a sortase-mediated reaction,

• wherein the sortase substrate comprises a structure of A1-Sp-M, in which
• A1 represents an agent, Sp represents one or more optional spacers, and M represents a sortase recognition motif comprising an unnatural amino acid located at position 5 from the direction of N-terminal to C-terminal of the sortase recognition motif, wherein the unnatural amino acid is an optionally substituted hydroxyl carboxylic acid having a formulae of CH2OH—(CH2)n—COOH, n being an integer from 0 to 3, preferably n=0.