Polyfunctional T Cell and Neutralizing Antibody Responses to ACAM2000<sup>TM</sup> Smallpox Vaccine Immunization in Primary-Vaccinated Individuals

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Abstract

Smallpox eradication was successful via prophylactic administration of live attenuated vaccinia virus. As a result of the discontinuation of the smallpox immunization program, many individuals are now susceptible to smallpox virus infection should it be used as a biological weapon. Presently, only individuals at high risk for exposure are required to receive smallpox vaccine, such as laboratory personnel that handle variola/vaccinia virus. This study endeavored to investigate a one-year period of vaccinia virus-specific T cell responses using polychromatic flow cytometry and neutralizing (Nt) antibody responses using plaque reduction neutralization test (PRNT) in individuals receiving primary immunization (n = 5) with ACAM2000TM smallpox vaccine. Functional and phenotypic profiles of vaccinia virus-specific T cell responses were characterized. Each single functional measurement {CD107a/b expression, production of interferon g (IFN-g), macrophage inflammatory protein 1b (MIP-1b), interleukin 2 (IL-2), and tumor necrosis factor a (TNF-a)} demonstrated that vaccinia virus-specific CD8+ T cells were functional at least one time point after vaccination (p ≤ 0.05). However, vaccinia virus-specific CD4+ T cells were functional only for MIP-1b production (p ≤ 0.05). Vaccinia virus-specific CD8+ T cells induced in these individuals showed increased polyfunctionality in at least 2 phenotypes relative to pre-vaccination (p ≤ 0.05). Although only three of five individuals (60%) showed positive Nt antibody (titer ≥ 20) at first month after vaccination, all five individuals (100%) demonstrated Nt antibody at 2 months, post-immunization. Interestingly, all vaccinees could retain the Nt antibody for 6 months after primary vaccination. In conclusion, ACAM2000TM smallpox vaccine induced both polyfunctional T cell-and Nt antibody-responses in primary immunized individuals.

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