Original sentence: Intracellular antigens are produced by viruses replicating within a host cell.The host cell digests pathogen-associated cytoplasmic proteins (in addition to endogenous self-proteins, as well as tumor-associated antigens) by a specialized enzyme complex, the proteasome into small peptides
Edited sentence: Intracellular antigens are mainly produced by viruses replicating within a host cell, though antigens here can also derive from cytoplasmic bacteria or the host cell's own proteins.The host cell digests cytoplasmic proteins by a specialized enzyme complex, the proteasome into small peptides