Detection and Quantification of CD4(+) T Cells with Specificity for a New Major Histocompatibility Complex Class II-Restricted Influenza A Virus Matrix Protein Epitope in Peripheral Blood of Influenza Patients

FVFTLTVPS was identified as the core sequence of a new major histocompatibility complex class II-restricted T-cell epitope of influenza virus matrix protein. Epitope-specific CD4(+) T cells were detected in the peripheral blood of patients with frequencies of up to 0.94%, depending on the number of additional terminal amino acids.