Natural Antibody Contributes to Host Defense against an Attenuated Brucella abortus virB Mutant

Abstract

Brucella abortus is an intracellular pathogen that persists within phagocytic cells of the reticuloendothelial system. To identify in vivo interactions between B. abortus and the host that lead to persistent infection, we studied the persistence of B. abortus and an isogenic virB mutant deficient in the VirB type IV secretion system (T4SS) in knockout mice. In contrast to control mice, mice lacking B cells (Igh6 ؊/؊ ) were permissive for infection with the attenuated virB mutant. To determine the basis for this phenotype, we characterized immune functions of Igh6 ؊/؊ mice in the context of B. abortus infection. Igh6 ؊/؊ mice had greater numbers of extracellular bacteria in the spleen and increased early expression of proinflammatory cytokines during B. abortus infection. Further, a virB mutant, despite its wild-type level of survival, failed to elicit microgranuloma formation in the spleens of Igh6 ؊/؊ mice, suggesting a requirement for the T4SS to elicit this pathological change. Passive transfer of immunoglobulin G from naïve mice restored the ability of Igh6 ؊/؊ mice to control the persistence of the virB mutant by a complement-independent mechanism. Further, adoptive transfer of CD11b ؉ cells from C57BL/6 mice to Igh6 ؊/؊ mice restored the ability of the knockout mice to limit the replication of the virB mutant in the spleen, suggesting that the Igh6 ؊/؊ mutation affects phagocyte function and that phagocyte function can be restored by natural antibody.Human brucellosis is a febrile disease resulting from the transmission of Brucella abortus, B. suis, B. melitensis, or B. canis from its respective zoonotic reservoir in cattle, swine, goats and sheep, or dogs (47). These pathogens are endemic in many areas of the world, including Central and South America, the Mediterranean, and Central Asia, and are responsible for an estimated 500,000 new brucellosis cases each year (1). In human brucellosis, as well as in the zoonotic reservoir species, bacteria may persist for long periods of time in the reticuloendothelial system (7). This aspect of infection can be modeled in the mouse, which has been used to identify and characterize the virulence factors involved in the systemic persistence of Brucella spp. (2,22).One essential virulence factor of the human pathogenic Brucella species is the type IV secretion system (T4SS) encoded by the virB locus on chromosome II (18,29,40). The T4SS of Brucella spp., similar to those of other bacterial pathogens, mediates the translocation of proteins into host cells; however, the functions of two newly identified B. abortus T4SS substrates, VceA and VceC, is not yet known (12,25,39,45,46). In cultured macrophages and dendritic cells (DC), the T4SS is essential for the intracellular replication and persistence of B. abortus (13,36,40). The T4SS mediates exclusion of late endosomal/lysosomal markers from the Brucella-containing vacuole and targeting of B. abortus to exit sites of the endoplasmic reticulum, where replication occurs (5, 6, 12, 41), suggesting that T4SS effectors are i...

PPARγ-Mediated Increase in Glucose Availability Sustains Chronic Brucella abortus Infection in Alternatively Activated Macrophages

“…Macrophages (CD11b + cells) were isolated from the spleens of B. abortus infected C57BL/6J, congenic Ccr2 −/− , Ifng −/− and Stat6 −/− mice with a MACS CD11b MicroBeads magnetic cell sorting kit from Miltenyi Biotech (Auburn, CA) by following the manufacturer’s instructions, as previously described (Rolán et al, 2009). Briefly, single cell suspension was prepared by gently teasing apart the spleens, followed passage of splenocytes through a 70-μm cell strainer and treatment with ACK buffer (Lonza, Walkersville, MD) to lyse red blood cells.…”

Crucial Role of Gamma Interferon-Producing CD4+Th1 Cells but Dispensable Function of CD8+T Cell, B Cell, Th2, and Th17 Responses in the Control of Brucella melitensis Infection in Mice

“…Several groups have previously analyzed this question, with contradictory results. Natural antibodies have been reported to favor Brucella control by facilitating its elimination by phagocytosis (61). Many serum transfer experiments have demonstrated a positive role of antibodies in protecting mice against Brucella infection (1,50,72).…”

“…The precise role of different components of adaptive immunity in the control of Brucella growth is quite controversial (1,3,6,24,48,51,52,55,58,60,61). Here, we compared the course of B. melitensis infection in the spleen of various C57BL/6 mice rendered genetically deficient for key elements of adaptive immune response: RAG (T and B cells),…”

Brucella melitensis Invades Murine Erythrocytes during Infection

“…Previously, several groups have analyzed the role of natural antibodies in an experimental model of Brucella, with contradictory results. Natural antibodies have been reported to promote the control of an attenuated Brucella mutant by facilitating its elimination by phagocytosis (45). Many serum transfer experiments have demonstrated that antibodies play a positive role in protecting mice against Brucella infection (46)(47)(48).…”