In this study we analyze the the immune evasive pathogen C. albicans. The secreted pH regulated antigen 1 (Pra1) is a protease and cleaves C3, the key molecule for cell opsonization. The paradox of this mechanism is that this cleavage of C3 produces a molecule that is very similar to the opsonin C3b. At a first glance it is not obvious why this protease has a regulatory influence on the system. This paradox is to be deciphered with a mathematical model.

Model of the 𝑃⁢𝑟⁢𝑎⁢1 interaction with the complement molecule 𝐶3 and its cleavage products. Fluid phase 𝑃⁢𝑟⁢𝑎⁢1𝑓 binds to 𝐶𝑓3 and cleaves it into fragments 𝐶3⁢𝑎⁢𝐿𝑓 and 𝐶3⁢𝑏⁢𝐿𝑓. In addition, $Pra1^f$ binds to the cleavage products 𝐶3⁢𝑎𝑓, 𝐶3⁢𝑏𝑓, 𝐶3⁢𝑎⁢𝐿𝑓 and 𝐶3⁢𝑏⁢𝐿𝑓 and blocks their effector function. The molecule 𝐶3⁢𝑏⁢𝐿𝑓 can bind to the cell surface like 𝐶3⁢𝑏𝑓 and there enhance complement activation via the 𝐶3-convertase of the alternative pathway or it is inactivated by regulators and cleaved to 𝑖⁢𝐶3⁢𝑏⁢𝐿𝑠.

For this purpose, the existing DynaCoSys model is extended to include the dynamics of Pra1 and its cleavage products. The deviation of opsonization on the surface and of the molecules in the fluid around the cell will be analyzed in comparison to a cell without Pra1 secretion.