Fungal cell walls are predominantly composed of
glucans,
mannans, and
chitin. Recognition of these
glycans by the innate immune system is a critical component of host defenses against the
mycoses.
Complement, an important arm of innate immunity, plays a significant role in fungal pathogenesis, especially the alternative pathway (AP). Here we determine that the
glycan monosaccharide composition and glycosidic linkages affect AP activation and C3 deposition. Furthermore,
properdin, a positive regulator of the AP, contributes to these functions. AP activation by
glycan particles that varied in composition and linkage was measured by C3a generation in serum treated with 10 mM
EGTA and 10 mM Mg(2+) (Mg-
EGTA-treated serum) (AP specific;
properdin functional) or Mg-
EGTA-treated serum that lacked functional
properdin. Particles that contained either β1→3 or β1→6
glucans or both generated large and similar amounts of C3a when the AP was intact. Blocking
properdin function resulted in 5- to 10-fold-less C3a production by particulate β1→3
glucans. However, particulate β1→6
glucans generated C3a via the AP only in the presence of intact
properdin. Interestingly,
zymosan and
glucan-
mannan particles (GMP), which contain both β-
glucans and
mannans, also required
properdin to generate C3a. The β1→4
glycans chitin and
chitosan minimally activated C3 even when
properdin was functional. Finally,
properdin binding to
glucan particles (GP) and
zymosan in serum required active C3.
Properdin colocalized with bound C3, suggesting that in the presence of serum,
properdin bound indirectly to
glycans through C3 convertases. These findings provide a better understanding of how
properdin facilitates AP activation by fungi through interaction with the cell wall components.
IMPORTANCE:
Invasive fungal infections have increased in incidence with the widespread use of immunosuppressive therapy and invasive procedures. Activation of the
complement system contributes to innate immunity against fungi by generating
chemoattractants that recruit white blood cells and by coating the pathogen with
complement fragments that "mark" them for phagocytosis. The fungal cell wall activates
complement in an antibody-independent manner through the alternative pathway (AP).
Properdin is a positive regulator of the AP. This study elucidates how the specificity of cell wall
glycan linkages affects AP activation and the role
properdin plays in this process. Particulate β1→3
glucans activated the AP even in the absence of
properdin, while β1→6
glucans required
properdin for AP activation. In contrast, the β1→4
glycans chitin and
chitosan failed to activate the AP. These findings enhance our mechanistic understanding of how fungi activate
complement and have implications for the use of
glycans in biomedical applications.