Antigen Presentation and TH Cell Proliferation

To understand which experiments would lead to T_H cell proliferation, we need to consider how T_H cells are activated. T helper (T_H) cells are a type of T cell that play a central role in the adaptive immune response. They require activation by antigen-presenting cells (APCs), such as macrophages.

The process of T_H cell activation involves:

1. Macrophages engulf and process antigens.
2. The processed antigen is broken down into peptides.
3. These peptides bind to MHC class II molecules inside the macrophage.
4. The MHC class II-peptide complexes are transported to the macrophage cell surface.
5. T_H cells recognize the specific peptide presented by the MHC class II molecule on the macrophage surface using their T cell receptor (TCR).
6. This recognition, along with co-stimulatory signals, leads to T_H cell activation and subsequent proliferation.

Paraformaldehyde is a fixative that cross-links proteins, essentially killing the cells and preventing further cellular processes like antigen processing or internal trafficking of molecules. However, it can preserve existing structures and surface molecules.

Analyzing the Antigen Experiments

Let's analyze each experiment described:

Experiment A: Macrophages treated with Antigen then incubated with T_H cells

• In this experiment, macrophages are live and functional when exposed to the antigen.
• They can take up the antigen, process it into peptides, and load these peptides onto MHC class II molecules.
• The MHC class II-peptide complexes are then presented on the macrophage surface.
• When incubated with antigen-primed T_H cells, these T_H cells can recognize the presented antigen peptides.
• This recognition should lead to T_H cell activation and proliferation.

Conclusion for A: T_H cell proliferation is expected.

Experiment B: Macrophages treated with Paraformaldehyde first, then with Antigen

• Macrophages are treated with paraformaldehyde before being exposed to the antigen.
• Paraformaldehyde fixes and kills the cells, preventing them from actively processing the antigen.
• Although antigen is present, the fixed macrophages cannot efficiently internalize, process, or load the antigen peptides onto MHC class II molecules.
• Without proper antigen processing and presentation, T_H cells will not receive the necessary signal for activation.

Conclusion for B: T_H cell proliferation is unlikely.

Experiment C: Macrophages treated with Paraformaldehyde first, then with Digested Antigen

• Macrophages are fixed with paraformaldehyde first, so they cannot process antigen themselves.
• However, they are then treated with *digested* antigen, meaning the antigen has already been broken down into peptides.
• Even though the macrophages are fixed, pre-processed peptides can often still bind to available MHC class II molecules, particularly those already on the cell surface or accessible within the fixed cell structure.
• If MHC class II-peptide complexes form and are presented on the surface, antigen-primed T_H cells can recognize them.

Conclusion for C: T_H cell proliferation is expected because processed peptides are provided externally and can likely bind to MHC II on fixed cells.

Experiment D: Macrophages treated with Antigen, then with Paraformaldehyde

• Macrophages are treated with antigen for an hour *before* fixation.
• This allows the live macrophages time to take up and process the antigen into peptides and load them onto MHC class II molecules.
• After this processing and presentation have likely occurred, the macrophages are fixed with paraformaldehyde.
• The fixation preserves the MHC class II-peptide complexes that are already present on the cell surface.
• When incubated with antigen-primed T_H cells, these T_H cells can recognize the stable MHC-peptide complexes on the fixed macrophage surface.

Conclusion for D: T_H cell proliferation is expected because antigen processing and presentation happen before fixation, preserving the presentation signal.

Summary of Experiment Outcomes

Based on the analysis:

• Experiment A: Leads to T_H cell proliferation.
• Experiment B: Does not lead to T_H cell proliferation.
• Experiment C: Leads to T_H cell proliferation.
• Experiment D: Leads to T_H cell proliferation.

Therefore, experiments A, C, and D would lead to T_H cell proliferation.