Figure 8.

Summary of Trojan horse mechanism (A). During melioidosis, microabscesses develop secondary to primary foci via hematogenous routes (step 1), and B. pseudomallei steadily persists in CD11b⁺Ly6C⁺ monocytes, CD11b⁺Ly6G⁺ neutrophils and CD11b⁺F4/80⁺ macrophages in the liver and spleen (step 2). After 4 d of infection, the bacteremia could disappear, but the liver and spleen become B. pseudomallei reservoirs. Once bacteremia recurs or persists (step 3), B. pseudomallei invades CD11b⁺CD34⁺ stem cells and CD11b⁺CD115⁺ phagocytic progenitors and differentiated CD11b⁺Ly6C⁺ monocytes, CD11b⁺Ly6G⁺ neutrophils, CD11b⁺F4/80⁺ macrophages and activated CD11b⁺CD14⁺ LPS-responding cells, as well as lymphoid–lineage CD11b⁺CD150⁺ cells in the BM (step 4). Prior to emigration, the inflamed CD11b⁺CCR2⁺, CD11b⁺CD62L⁺ and CD11b⁺CD31⁺ BM cells harboring B. pseudomallei mature (step 5). On d 21 post-infection, the inflamed Ly6C⁺CD62L⁺GFP⁺ cells become a predominant B. pseudomallei-loaded subpopulation, and the circulating monocyte-derived CD11b⁺Ly6C⁺ and CD11b⁺F4/80⁺ cells harboring B. pseudomallei are expanded (step 6). Acting as a Trojan horse, the infected Ly6C⁺CD62L⁺ cells cross the cerebral endothelium via selectin-mediated leukocyte migration, resulting in expansion of the circulating CD16/32⁺CD45^hi phagocytes in the BILs (step 7) and eventual B. pseudomallei colonization of the brain (step 8). Comparison of BALB/c and C57BL/6 mice with melioidosis (B). During early infection (by d 4 post-infection), the circulating CD16/32⁺CD45^hiGFP⁺ phagocytes rapidly infiltrate the brains of C57BL/6 and BALB/c mice; however, the growth of intracellular B. pseudomallei is restricted (Part 1). On d 14 post-infection (during late infection), CD16/32⁺CD45^loGFP⁺ microglia are expanded in the BILs in both BALB/c and C57BL/6 mice with melioidosis. However, in BALB/c mice with melioidosis, the CD16/32⁺CD45^hiGFP⁺ phagocytes are rapidly expanded in the BILs, eventually resulting in the growth of B. pseudomallei in the brain. However, CD16/32⁺CD45^hiGFP⁺ phagocytes are rarely found in C57BL/6 mice, and consequently, B. pseudomallei has difficultly colonizing C57BL/6 brains (Part 2). After adoptive transfer of B. pseudomallei-loaded splenic CD11b cells (see reference 20) or BM Ly6C cells, the bacterial loads in the recipient BALB/c brains increase. In contrast, the bacterial loads in the C57BL/6 recipient brains are decreased after adoptive transfer of sell^-/- Ly6C cells compared to adoptive transfer of sell^+/+ Ly6C cells. When anti-CD62 antibody-treated Ly6C cells infected with B. pseudomallei act as donors, bacterial colonization of recipient BALB/c and C57BL/6 brains is also depleted. Besides, bacterial colonization of both BALB/c and C57BL/6 brains is not induced by intravenous injection of free B. pseudomallei (Part 3).