The encapsulated strain TIGR4 of Streptococcus pneumoniae is phagocytosed but is resistant to intracellular killing by mouse microglia.
The polysaccharide capsule is a major virulence factor of Streptococcus pneumoniae as it confers resistance to phagocytosis. The encapsulated serotype 4 TIGR4 strain was shown to be efficiently phagocytosed by the mouse microglial cell line BV2, whereas the type 3 HB565 strain resisted phagocytosis. Comparing survival after uptake of TIGR4 or its unencapsulated derivative FP23 in gentamicin protection and phagolysosome maturation assays, it was shown that TIGR4 was protected from intracellular killing. Pneumococcal capsular genes were up-regulated in intracellular TIGR4 bacteria recovered from microglial cells. Actual presence of bacteria inside BV2 cells was confirmed by transmission electron microscopy (TEM) for both TIGR4 and FP23 strains, but typical phagosomes/phagolysosomes were detected only in cells infected with the unencapsulated strain. In a mouse model of meningitis based on intracranic inoculation of pneumococci, TIGR4 caused lethal meningitis with an LD(50) of 2 × 10² CFU, whereas the LD(50) for the unencapsulated FP23 was greater than 10⁷ CFU. Phagocytosis of TIGR4 by microglia was also demonstrated by TEM and immunohistochemistry on brain samples from infected mice. The results indicate that encapsulation does not protect the TIGR4 strain from phagocytosis by microglia, while it affords resistance to intracellular killing.
Peppoloni
Samuele
S
Department of Public Health Sciences, University of Modena and Reggio Emilia (Unimore), 41125 Modena, Italy. samuele.peppoloni@unimore.it
Ricci
Susanna
S
Orsi
Carlotta F
CF
Colombari
Bruna
B
De Santi
Maria Margherita
MM
Messinò
Massimino
M
Fabio
Giuliana
G
Zanardi
Alessio
A
Righi
Elena
E
Braione
Velia
V
Tripodi
Sergio
S
Chiavolini
Damiana
D
Cintorino
Marcella
M
Zoli
Michele
M
Oggioni
Marco Rinaldo
MR
Blasi
Elisabetta
E
Pozzi
Gianni
G
D000818:Animals; D016667:Bacterial Capsules / Q000276:immunology / Q000378:metabolism*; D001921:Brain / Q000382:microbiology / Q000473:pathology; D002478:Cells, Cultured; D004195:Disease Models, Animal; D020869:Gene Expression Profiling; D015964:Gene Expression Regulation, Bacterial; D005798:Genes, Bacterial; D007150:Immunohistochemistry; D007928:Lethal Dose 50; D016920:Meningitis, Bacterial; D051379:Mice; D050296:Microbial Viability; D017628:Microglia / Q000276:immunology / Q000382:microbiology*; D046529:Microscopy, Electron, Transmission; D010587:Phagocytosis; D013296:Streptococcus pneumoniae / Q000276:immunology / Q000472:pathogenicity*; D016019:Survival Analysis; D014774:Virulence; D037521:Virulence Factors / Q000276:immunology / Q000378:metabolism*
D037521:Virulence Factors