73. In a laboratory experiment it was
    observed that both ‘Virus A’ and ‘Virus B’ could infect a mammalian host cell, when infected individually. Interestingly, if the cell
    were first infected with Virus A (with large MOI), Virus B failed to infect the same cell. If the Virus B (with large MOI) is added first
    followed by Virus A, both the virus can infect the cells. However, infection with ‘Virus A’ was found to be in lesser extent. Considering X and Y are the receptors/co-receptors which may be involved for the virus entry, following are few possibilities that can explain the observation.
    A. ‘Virus A’ uses ‘X’ as receptor and Y as co-receptor.
    B. ‘Virus B’ uses exclusively ‘Y’ as receptor for entry:
    C. Both ‘Virus A’ and ‘Virus B’ need X as
    receptor.
    Choose the option with all correct
    statements.
    (1) A, B and C
    (2) A and B
    (3) B and C
    (4) A and C

Virus Receptor Usage and Competitive Infection: Understanding Virus A and Virus B Interactions

Introduction

In experiments with two viruses, Virus A and Virus B, both individually can infect a mammalian cell. However, sequential infection reveals interesting competition effects:

• If Virus A infects first at a high multiplicity of infection (MOI), Virus B fails to infect the cell.

• If Virus B infects first at high MOI followed by Virus A, both viruses infect, but Virus A infection is reduced.

This suggests interplay involving shared or distinct receptors/co-receptors, named here as X and Y.

Analyzing Proposed Statements

1. Statement A: Virus A uses receptor X and co-receptor Y.

   • This explains Virus A’s infection requiring both receptors; occupation of X and Y by Virus A would prevent Virus B access if Virus B depends on Y or X.

2. Statement B: Virus B uses exclusively Y as a receptor for entry.

   • This fits the observation that Virus B infection fails if receptor Y is occupied by Virus A first (high MOI); thus Virus B needs Y but not X.

3. Statement C: Both Virus A and Virus B need X as receptor.

   • Conflicts with statement B and the infection outcomes. If both use X, Virus B infection would likely fail when Virus A is first inoculated due to receptor saturation. However, Virus B infection depends on Y alone according to B, so this is unlikely.

Matching Data to Options

• Experiments show Virus B infects after Virus A only when Virus B is first; meaning receptor Y is less available after Virus A.

• Virus B requires Y only; Virus A requires X and Y. This fits statements A and B.

Statement C is contradictory to B and inconsistent with observations.

Conclusion

The correct combination is (2) A and B, as both are consistent with the observed infection patterns.