O papel da proteína adaptadora fator de diferenciação miéloide 88 (MyD88) e dos Receptores do tipo Toll (TLRs)na ativação e maturação de células dentríticas quando estimuladas por componentes bacterianos da Brucella abortus

Abstract

Brucella abortus is a gram-negative bacteria, intracellular pathogen, which causes undulant fever in humans and infertility among animals, resulting in serious economic losses. Brucella recognition mediated by pattern recognition receptors (PRRs), such as Toll-like receptors (TLRs), is an important step to establish immune responses against this organism as it culminates in a strong Th1 polarization, desired for an efficient bacterial clearance (Golding et al., 2001). TLR4 (Campos et al., 2004), TLR2 (Giambartolomei et al., 2004) and TLR9 (Huang et al., 2005) participates in in vitro recognition of Brucella abortus. However, TLR 2 and TLR4 are not required for Brucella clearance in vivo. Nonetheless, the molecule MyD88, major downstream adaptor of TLRs, is required for in vivo control of Brucella infections (Weiss et al., 2005). Dendritic cells (DCs), a potential reservoir of Brucella during infections (Billard et al., 2005), are the sole producers of IL-12 in the spleen of mice stimulated with heat-killed Brucella (HKBa) (Huang et al., 2001), which turns them critical cells to sustain efficient responses against the pathogen. The present work evaluated the role of MyD88 and TLRs in the DC recognition of Brucella. Surprisingly, the cytokine production and the maturation of bone marrow dendritic cells (BMDCs) in response to HKBa were completely impaired in absence of MyD88. BMDCs from TLR4-/- mice were normal responders to Brucella stimuli, on the other hand, BMDCs from TLR2-/- mice could neither maturate nor induce TNF- and IL-12 secretion in response to HKBa. Brucella TLR2-dependent IL-12 response in BMDCs was confirmed by inhibiting TLR2 activity by neutralizing this receptor in BMDCs with specific antibodies. Interestingly, macrophages showed different TLR requirements for responses against Brucella as TNF- secretion induced by HKBa was TLR2- dependent, but IL-12 responses were neither TLR2, nor TLR4 dependent, in accordance to other groups (Huang et al., 2003). The maturation of splenic DCs in vivo after administration of Brucella stimuli was MyD88-dependent, but TLR2- independent. The controversy of in vivo and in vitro data is, probably, due to different responses of DC subtypes analyzed. The lack of a major role for TLR4 mediated responses against Brucella can be explained by the less immunogenic, non-classical, lipopolysaccharide displayed by Brucella (Lapaque et al., 2005). The scenery emerging of this study is: Even though TLR2 has a role in Brucella abortus cytokine responses in some DCs subtypes, splenic DCs recognize Brucella through other receptor, probably TLR9. This recognition leads to Th1 cells differentiation and secretion of IFN-, which, in turn, activates downstream responses.