Pathogenic Bacteria Immune Evasion

Pathogenic bacteria have evolved various strategies to evade the host immune system, including mimicking host proteins. This question explores how bacteria mimicking human complement regulatory proteins might affect complement activation and bacterial survival.

The complement system is a critical part of innate immunity that helps clear pathogens. It can be activated via three main pathways: classical, alternative, and lectin pathways. All pathways converge on the formation of C3 convertase, which cleaves C3 into C3a and C3b. C3b deposition on the pathogen surface leads to opsonization (marking for phagocytosis), formation of C5 convertase, and eventually the formation of the Membrane Attack Complex (MAC), which lyses the cell.

Host cells are protected from inappropriate complement activation by various regulatory proteins like Decay Accelerating Factor (DAF), Complement Receptor 1 (CR1), Factor I, and Protein S.

Understanding Mimicked Proteins

• Decay Accelerating Factor (DAF): A protein on host cell surfaces that prevents the formation of C3 and C5 convertases or accelerates their decay. It works in all three pathways (classical, alternative, lectin).
• Complement Receptor 1 (CR1): A protein on host cell surfaces that acts as a cofactor for Factor I in the cleavage of C3b and C4b, thus inhibiting C3 and C5 convertase formation. It also promotes convertase decay.
• Factor I: A serine protease that cleaves C3b (with cofactors like Factor H, CR1, MCP) and C4b (with cofactors like C4BP, CR1, MCP). This cleavage inactivates C3b and C4b, preventing convertase formation.
• Protein S: A soluble plasma protein that binds to the soluble C5b-C6-C7 complex and the SC5b-9 complex, preventing the insertion of C7 and the MAC into host cell membranes. It primarily inhibits MAC formation.

Analyzing Bacterial Mimicry Strategies

Let's look at how each bacterium uses mimicry:

Evaluating the Statements

Now, let's examine each statement based on our understanding:

1. Statement 1: Bacterium X will prevent formation of C3 convertase on its surface by alternate and classical pathways.

   Bacterium X mimics DAF and CR1. DAF and CR1 both inhibit C3 convertase formation and promote its decay in the classical, alternative, and lectin pathways. Therefore, mimicking these proteins will indeed prevent C3 convertase formation on the bacterial surface via these pathways. This statement is likely correct.

2. Statement 2: Bacterium Y will prevent formation of C3 convertase on its surface by lectin pathway.

   Bacterium Y secretes a protein mimicking human protein S. Protein S functions downstream of C3 convertase formation. It inhibits the formation of the Membrane Attack Complex (MAC) by binding to the C5b-9 complex. Protein S has no known role in preventing C3 convertase formation, whether initiated by the classical, alternative, or lectin pathway. This statement is likely incorrect.

3. Statement 3: Bacterium Z will be susceptible to complement attack by Membrane Attack Complex (MAC) despite secreting Factor I-like protein to cleave C3b and C4b.

   Bacterium Z secretes a protein mimicking Factor I activity. Factor I cleaves C3b and C4b, which helps prevent C3 convertase formation. However, if some C3 convertase does form and C5 convertase activity occurs, leading to C5b generation and subsequent MAC formation, Factor I cannot prevent MAC assembly or insertion. Therefore, the bacterium could still be susceptible to MAC attack if complement activation proceeds beyond the C5 step. This statement is likely correct.

4. Statement 4: Bacterium Y will prevent formation of Membrane Attack Complex (MAC) on its surface.

   Bacterium Y mimics human protein S. As discussed, protein S inhibits the formation of the MAC by binding to intermediates of the MAC complex. Mimicking protein S would effectively prevent MAC formation on the bacterial surface. This statement is likely correct.

Conclusion

Based on the analysis, the statement that is INCORRECT is Statement 2, which claims that Bacterium Y will prevent formation of C3 convertase on its surface by the lectin pathway. Bacterium Y mimics protein S, which specifically inhibits MAC formation, not C3 convertase formation.