Strains were evaluated in vitro and in mice. Twenty-two healthy volunteers received single oral doses of either strain in a physiological study of safety, shedding, and immunogenicity. Volunteers were observed in the hospital for seven days and had daily blood cultures, routine safety blood tests (complete blood count with differential; hepatic and renal function), and fecal cultures; none had fever, positive blood cultures, prolonged shedding, or serious or unexpected events. Four of 12 volunteers who received the actA/plcB-deleted strain had minor, transient, asymptomatic serum transaminase elevations (maximum increase 1.4×
upper normal). Six of six volunteers who received ≥4 × 109 colony forming units had detectable mucosal immune responses to listerial antigens, PF-02341066 in vivo but not to the vectored influenza antigen. Approximately half the volunteers had modest interferon-γ ELISpot responses to a complex listerial antigen, but none had increases over their baseline responses to the influenza antigen. Comparison with prior work suggests that foreign antigen expression, and perhaps also freezing, may adversely affect the organisms’ immunogenicity. Live attenuated bacteria expressing foreign antigens present a promising approach in the development of human immunotherapies and vaccines. Favorable attributes of
Listeria monocytogenes include its innate immunostimulatory EX-527 properties, efficient cytosolic entry of antigen presenting cells, and its ability to stimulate vigorous CD4 and CD8 T cell responses. L. monocytogenes antigen processing and presentation via
the major histocompatability complex class I pathway makes it an attractive vector for delivery of viral and cancer antigens. Animal studies show that L. monocytogenes vectors can generate T cell-mediated immune responses to lymphocytic choriomeningitis virus, influenza virus, HIV-1 Gag, and human papilloma virus-16 antigens (1–5). Despite these promising results, there are safety concerns related to using recombinant wild type new L. monocytogenes as a vaccine vector due to the high degree of morbidity and mortality that results from naturally acquired infection. Several groups have developed strategies for attenuating L. monocytogenes by deletion of specific virulence genes (1, 6–8) and two strains have been evaluated in published human studies: ΔactA/plcB (9) and ΔprfA (10) via oral and parenteral routes, respectively. In our prior oral clinical study, 20 healthy adult volunteers received escalating single doses of live attenuated L. monocytogenesΔactA/plcB safely. No individual experienced a serious adverse event. Three of 20 individuals had mild elevations in serum transaminases (maximum 2.5× upper limit of normal) that were temporally associated with vaccination and could not be otherwise explained. Subsequently, another biotechnology group developed an L.