It has been shown that BALB/c mice are very efficient at controlling infection with S. pyogenes
). The resolution of streptococcal infection in these mice was dependent on innate immune mechanisms, as shown by the capacity of immunodeficient SCID BALB/c mice to clear infection (18
). It was also shown that resistance to S. pyogenes
in BALB/c mice was expressed very early during the infection process (18
), suggesting the participation of resident immune mechanisms in these events. In this study, we sought to elucidate the contribution of macrophages to resistance expressed by BALB/c mice to infection with S. pyogenes.
Resident macrophages constitute the primary line of innate defense against most bacterial pathogens (2
). Macrophages are found in all body tissues, and they function as sentinels capable of rapidly recognizing and internalizing bacterial pathogens, and they also recruit other immune cells to the site of infection by the production of chemokines and acute-phase proteins (3
). In this study, we sought to elucidate the contribution of macrophages to resistance expressed by BALB/c mice to infection with S. pyogenes.
The critical role of macrophages during infection with S. pyogenes was demonstrated here by the increased susceptibility of macrophage-depleted mice. Macrophage-depleted mice were unable to recover from a sublethal dose of S. pyogenes, with 100% of the macrophage-depleted animals dead by day 4 postinoculation, whereas 90% of the control animals survived infection. This increased mortality was accompanied by enhanced bacterial loads in the blood and systemic organs. Interestingly, increased susceptibility to infection was observed only when depletion of macrophages was performed prior to or during the first 24 h postinoculation, which indicates that macrophages might be required for resistance against S. pyogenes during the very early phase of the infection.
The possibility that macrophages may play an important role in clearing streptococci was previously suggested by Ji and coworkers (13
). In that study, a mutant strain of S. pyogenes
deficient in C5a peptidase was found to be more efficiently eliminated than the wild-type strain in spite of attracting higher number of PMNs to the site of infection and being resistant to the phagocytic activity of these cells in vitro (13
The mechanisms underlying the protective function of macrophages seem to be dependent on the ability of these phagocytic cells to efficiently take up and kill S. pyogenes, as demonstrated by (i) double-immunofluorescence microscopy showing that most of the microorganisms associated with macrophages were localized intracellularly, (ii) survival assays showing that macrophages are capable of killing S. pyogenes at early stages of ingestion, and (iii) electron microscopy studies showing intraphagosomal destruction of streptococcal microorganisms.
The role of macrophages in host defense has been shown to be particularly important early in the host-pathogen encounter, when specific immunity has not yet developed. Several pathogens express surface domains that are recognized by macrophages in the absence of specific antibodies (24
). These interactions have been shown to play an important role in the phagocytosis of Listeria monocytogenes
, Pseudomonas aeruginosa
, Neisseria meningitidis
, group B streptococci, and mycobacteria, among others (4
Two principal mechanisms of phagocytosis have been described, opsonin-dependent phagocytosis (37
) and opsonin-independent phagocytosis (24
). In opsonin-dependent phagocytosis, immunoglobulins or complement molecules bind to microorganisms, thereby promoting ingestion via Fcγ or complement receptor on phagocytic cells (37
). In contrast, in opsonin-independent phagocytosis, ligands on the surfaces of microorganisms are directly recognized by receptors on the surfaces of phagocytes (24
). Which of these mechanisms is preferentially used by resident mouse macrophages to take up S. pyogenes
during early infection is under investigation.
In addition to a direct killing activity, macrophages can indirectly contribute to control of streptococcal infections by promoting the influx of other inflammatory cells into the focus of infection. The capacity of macrophages to promote chemotaxis in response to S. pyogenes
is supported by several in vitro studies showing the ability of the microorganism to activate transcription factors involved in cytokine signaling and to induce the expression of chemokines in human macrophages (19
). The cooperation between macrophages and PMNs may be important for the final outcome of the infection.
High variability in the capacities of genetically different mouse strains to resolve S. pyogenes
infection has been also reported (9
). Although the specific host characteristics that contribute to the observed differences in the resolution of streptococcal infection are still unknown, the inability to eliminate bacteria, leading to an overproduction of inflammatory mediators, was the main mechanism underlying the more susceptible phenotype (9
). The possibility that host genetic factors may determine the capacity of resident macrophages to reduce the number of bacteria during early infection should be addressed in further studies.
In summary, the results obtained in this study clearly indicate that macrophages are central determinants of the course of infection caused by S. pyogenes. Further studies are required to identify the bacterial determinants and their counterpart receptors on macrophages involved in phagocytosis and/or the release of inflammatory mediators.