The Apa Protein of Mycobacterium tuberculosis Stimulates Gamma Interferon-Secreting CD4 + and CD8 + T Cells from Purified Protein Derivative-Positive Individuals and Affords Protection in a Guinea Pig Model

Abstract

ABSTRACT The search to identify Mycobacterium tuberculosis antigens capable of conferring protective immunity against tuberculosis has received a boost owing to the resurgence of tuberculosis over the past two decades. It has long been recognized that lymphoid cells are required for protection against M. tuberculosis . While traditionally the CD4 + populations of T cells were believed to predominantly serve this protective function, a pivotal role for CD8 + T cells in this task has been increasingly appreciated. We show that the 50- to 55-kDa Apa protein, specified by the Rv1860 gene of M. tuberculosis , can elicit both lymphoproliferative response and gamma interferon (IFN-γ) production from peripheral blood mononuclear cells (PBMC) of purified protein derivative (PPD)-positive individuals, with significant differences recorded in the levels of responsiveness between PPD-positive healthy controls and pulmonary tuberculosis patients. Flow cytometric analysis of whole blood stimulated with the recombinant Apa protein revealed a sizeable proportion of CD8 + T cells in addition to CD4 + T cells contributing to IFN-γ secretion. PBMC responding to the Apa protein produced no interleukin-4, revealing a Th1 phenotype. A DNA vaccine and a poxvirus recombinant expressing the Apa protein were constructed and tested for their ability to protect immunized guinea pigs against a challenge dose of virulent M. tuberculosis . Although the DNA vaccine afforded little protection, the poxvirus recombinant boost after DNA vaccine priming conferred a significant level of protective immunity, bringing about a considerable reduction in mycobacterial counts from the challenge bacilli in spleens of immunized guinea pigs, a result comparable to that achieved by BCG vaccination.