Fig 1.
The biochemical reactions of the alternative pathway.
Fluid state activation of the alternative pathway leads to C3b that can indiscriminately bind to host or pathogen surface. On the right (pathogen), complement activity propagates without the disruption of regulatory proteins. On the left (host), plasma proteins in conjunction with surface-bound regulators actively inhibit complement propagation by disrupting complement interaction at different points of the alternative pathway. Abbreviations: C3, complement component 3; C3(H2O), thioester-hydrolyzed form of C3; C3(H2O)Bb, initial C3 convertase of AP; C3a, anaphylatoxin fragment from C3; C3b, activated fragment from C3; iC3b, inactivated C3b; nfC3b, nascent fluid phase C3b; nhC3b, nascent host C3b; npC3b, nascent pathogen C3b; fC3b, fluid phase C3b; fC3bBb, fluid phase C3 convertase, C3bBb, C3 convertase; C3bBbP, C3 convertase with properdin; C3bBbP*, C3 convertase with properdin* released from neutrophils; C5, complement component 5; C5a anaphylatoxin fragment from C5; C5b, activated fragment from C5; C3bBbC3b, C5 convertase; FB, Factor B; FD, Factor D; FI, Factor I; CR1, Complement Receptor 1; DAF, Decay Accelerating Factor; Vn, vitronectin; Cn, clusterin; CD59, Protectin. Lines and colors: solid lines with an arrow tip denote activation and dashed lines with a straight tip denote inhibition. Red line color denotes inhibition. The rest of line colors denote clusters of reactions involving different C3b molecules, i.e. nhC3b (orange), nfC3b (light blue), npC3b (purple), fC3b (dark blue).
Fig 2.
Complement deposition on pathogen and host surfaces.
Saturation of pathogen surface is reached in 54 minutes with complement proteins such as C3b, iC3b, iC3bP (pathogen), C3dg, C3/C5 convertases, properdin (P*), MAC, and intermediates such as C3bB, C3bBP, C5b6-7, C5b6-8, and others. Propagation of the alternative pathway is inhibited on host cells resulting in complement proteins occupying significantly less than 1 percent.
Fig 3.
Concentration of assembled fluid phase convertases, C3(H2O)Bb and C3bBb, of the alternative pathway.
The time profiles show that the concentration of C3(H2O)Bb increases significantly within 60 minutes compared to the concentration of C3bBb.
Fig 4.
Formation of C3 cleaving enzymes on the surface of host cells and pathogens.
C3bBbP* occupies 2.7 percent of the pathogen surface in 51 minutes. The asterisk denotes properdin released from neutrophils. The time profile of C3bBbP* shows a curve with a lag phase followed by an accelerated production (surface occupational) phase. The production of C3bBb and C3bBbP occupies significantly less than 1 percent on pathogens and even less on host cells. The orange and green lines are obscured by the blue line in the figure.
Fig 5.
Time profile for the production of C3a and C5a.
(A) The response generated for C3a shows a lag phase that is followed by an accelerated production phase. In 60 minutes, the amount of C3a produced is 108 times greater than that of C5a. (B) Zoom-in of panel (A) to show the time profile of C5a.
Fig 6.
Time profile production for cleavage products of C3b. iC3b and C3dg.
(A) iC3b. (B) C3dg. Both proteins take significantly less than 1 percent of pathogen and host cell surfaces.
Fig 7.
Formation of MAC pores is characterized by three phases: lag phase, production phase, and a steady state phase.
The first 22 minutes present a lag phase that is followed by a rapid production of MAC pores, which plateaus in 54 minutes and occupies 3.3 percent of the pathogen surface. However, on host cells the MAC pores take significantly less than 1 percent.
Fig 8.
Time course for properdin (P*) as a stabilizer and not an initiator of the alternative pathway.
(A) More than 99 percent of pathogen surface remains unoccupied by complement components. (B, C) The C3 convertases and MAC pores occupy significantly less than 1 percent on pathogens and even less on host cells.
Fig 9.
Time profiles for complement regulators Factor H, CR1, DAF, vitronectin, clusterin, and CD59.
(A, B) The concentrations of Factor H and CR1 decrease, with Factor H showing initially a linear like response. The initial response of Factor H signifies the regulation taking root at the start of the AP by inhibiting C3(H2O) and C3(H2O)Bb. (C) The concentration of DAF remains constant and does not change, which highlights the rapid deactivation of any C3b by FH and CR1. (D,E) Vitronectin and clusterin experience a long delay of 52 and 51 minutes respectively before their concentration starts decreasing. Their regulatory component takes root at the formation of fluid C5b-7, which comes at a later stage of complement activation. (F) The response generated for CD59 is the same as that of DAF (no change in concentration). This is expected since DAF remains the same (inhibition of C3 convertase formation), the terminal cascade for MAC pore formation will not take root because formation C5 convertases will also be inhibited.
Table 1.
Results of multi-parametric sensitivity analysis to initial complement concentrations.
In both pathogens and host cells, C3 strongly influenced the model output. Large values of K-S (Kolmogorov-Smirnov) test indicate that the model output is sensitive to the given parameter variations.
Fig 10.
Results of multi-parametric sensitivity analysis to kinetic parameter values.
The most sensitive rates on the surface of pathogens (red) are , and
, which represent the rate at which properdin associates to the surface and the rate at which it is released from neutrophils, respectively. While most kinetic parameter are mildly sensitive on host cells (blue),
and
, which represent association of Factor H to hydrolyzed C3 and conversion rate of nfC3b to fC3b, respectively, are more sensitive to parameter variations. The Michaelis–Menten kinetic parameters, kcat and Km of the fluid phase convertase C3(H2O)Bb are also sensitive.