Caspase-2-Dependent Dendritic Cell Death, Maturation, and Priming of T Cells in Response to Brucella abortus Infection

Abstract

Smooth virulent Brucella abortus strain 2308 (S2308) causes zoonotic brucellosis in cattle and humans. Rough B. abortus strain RB51, derived from S2308, is a live attenuated cattle vaccine strain licensed in the USA and many other countries. Our previous report indicated that RB51, but not S2308, induces a caspase-2-dependent apoptotic and necrotic macrophage cell death. Dendritic cells (DCs) are professional antigen presenting cells critical for bridging innate and adaptive immune responses. In contrast to Brucella-infected macrophages, here we report that S2308 induced higher levels of apoptotic and necrotic cell death in wild type bone marrow-derived DCs (WT BMDCs) than RB51. The RB51 and S2308-induced BMDC cell death was regulated by caspase-2, indicated by the minimal cell death in RB51 and S2308-infected BMDCs isolated from caspase-2 knockout mice (Casp2KO BMDCs). More S2308 bacteria were taken up by Casp2KO BMDCs than wild type BMDCs. Higher levels of S2308 and RB51 cells were found in infected Casp2KO BMDCs compared to infected WT BMDCs at different time points. RB51-infected wild type BMDCs were mature and activated as shown by significantly up-regulated expression of CD40, CD80, CD86, MHC-I, and MHC-II. RB51 induced the production of cytokines TNF-α, IL-6, IFN-γ and IL12/IL23p40 in infected BMDCs. RB51-infected WT BMDCs also stimulated the proliferation of CD4+ and CD8+ T cells compared to uninfected WT BMDCs. However, the maturation, activation, and cytokine secretion are significantly impaired in Casp2KO BMDCs infected with RB51 or Salmonella (control). S2308-infected WT and Casp2KO BMDCs were not activated and could not induce cytokine production. These results demonstrated that virulent smooth strain S2308 induced more apoptotic and necrotic dendritic cell death than live attenuated rough vaccine strain RB51; however, RB51, but not its parent strain S2308, induced caspase-2-mediated DC maturation, cytokine production, antigen presentation, and T cell priming.

Mechanism of Asp24 Upregulation in Brucella abortus Rough Mutant with a Disrupted O-Antigen Export System and Effect of Asp24 in Bacterial Intracellular Survival

“…Disruption of the rfbE or rfbD gene induces the transformation of smooth-type Brucella strains into rough-type strains due to the creation of an incomplete LPS lacking O-antigen (19). The ⌬rfbE mutant shows a rough phenotype, which enhances internalization into the host cell and induces macrophage oncosis and necrosis, like the case with other Brucella rough mutants (29,34,35). The mutant also has reduced survival in the host cell and fails to establish chronic infection in a mouse model (23).…”

Immune response triggered by Brucella abortus following infection or vaccination

“…It has been shown that cattle DCs are resistant to B. abortus infection, in spite of exhibiting some signs of maturational and activation impairment and lack of up-regulation of co-stimulatory molecules and IL-12p40 after infection [37]. Mice and human DCs are susceptible to B. abortus infection [36,38]. These differences in susceptibility of DCs among hosts may be related to the differences observed in the progression of the disease, since cattle is more able to control the infection, showing less clinical signs compared to humans and some mice strains.…”

“…These differences in susceptibility of DCs among hosts may be related to the differences observed in the progression of the disease, since cattle is more able to control the infection, showing less clinical signs compared to humans and some mice strains. However, rough B. abortus strains are able to induce higher phenotypic and functional maturation of human and murine DC cells, characterized by IL-12 and TNF-␣ secretion, and naive CD4 T-lymphocytes stimulation, compared to smooth strains [38][39][40]. Higher exposition of outer membrane proteins (Omp) in rough strains, compared to smooth strains, has been indicated as responsible for the stronger DC maturation in infection by rough strains [39].…”

“…Higher exposition of outer membrane proteins (Omp) in rough strains, compared to smooth strains, has been indicated as responsible for the stronger DC maturation in infection by rough strains [39]. The maturation and activation of DCs along with cytokine secretion after B. abortus infection seem to be dependent of caspase-2 and TLR6 [38,41]. Caspase-2 plays different roles during rough and smooth strains infections, being critical to mouse DC maturation and cytokine production in rough strains infection, whereas in infection by smooth strains it promotes the cell death, favoring bacterial dissemination [38].…”

“…The maturation and activation of DCs along with cytokine secretion after B. abortus infection seem to be dependent of caspase-2 and TLR6 [38,41]. Caspase-2 plays different roles during rough and smooth strains infections, being critical to mouse DC maturation and cytokine production in rough strains infection, whereas in infection by smooth strains it promotes the cell death, favoring bacterial dissemination [38]. TLR6 is required by mouse DC to induce TNF-␣ and IL-12 [41].…”