Antigen Presentation & Monoclonal Antibodies

SEO Title: MHC Molecules & Hybridoma Technology | Joyful CSIR-NET Notes

Search Meta Description: Have fun mastering MHC & Monoclonal Antibodies! High-yield notes on MHC I vs II, TAP, Invariant Chain, HLA, and the complete Hybridoma/HAT medium pathway for CSIR NET Life Sciences.

MHC MOLECULES & HYBRIDOMA TECH
Chapter 15: Antigen Presentation & Monoclonal Antibodies

Welcome to Chapter 15! You are absolutely mastering Immunology! 🌟
The Major Histocompatibility Complex (MHC) is the cell's display window—it tells the immune system what is happening inside. Examiners will heavily test your knowledge of MHC Class I vs. Class II processing pathways (Proteasome vs. Lysosome, TAP vs. Invariant Chain). The second half of this chapter covers a Nobel Prize-winning laboratory technique: Hybridoma Technology (HAT selection). Let's decode these pathways so you can ace those 4-mark Part C questions!

Quick Navigation Index

• 1. Introduction to MHC (HLA Complex)
• 2. MHC Class I (Endogenous Pathway)
• 3. MHC Class II (Exogenous Pathway)
• 4. Master Comparison: MHC I vs. MHC II
• 5. MHC Genetics (Polymorphism & Haplotypes)
• 6. Hybridoma Technology (Monoclonal Antibodies)
• 7. The Magic of HAT Medium
• 8. High-Yield CSIR-NET / GATE Memory Tricks
• 9. Fun & High-Yield Master Quiz!

---

1. Introduction to MHC (HLA Complex)

T-cells are completely blind to free-floating antigens. They can only "see" an antigen if it is chopped up and presented on a silver platter. That platter is the Major Histocompatibility Complex (MHC).

In humans, MHC is called the HLA Complex (Human Leukocyte Antigen). It is located on Chromosome 6.

Class	Human Genes (HLA)	Expressed On	Recognized By
Class I	HLA-A, B, C	ALL nucleated cells (Not RBCs!)	CD8+ Cytotoxic T-cells
Class II	HLA-DP, DQ, DR	Professional APCs only (DCs, Macrophages, B-cells)	CD4+ Helper T-cells
Class III	Complement & Cytokines	Secreted proteins	Does NOT present antigen. Produces C2, C4, TNF-α.

---

2. MHC Class I (The Endogenous Pathway)

MHC-I is like a security camera for the inside of the cell. If a virus infects a cell, the cell chops up the viral proteins and puts them on MHC-I to tell CD8+ T-cells: "I am infected, kill me!"

• Structure: One large heavy α chain (domains α1, α2, α3) associated with a tiny non-MHC protein called β2-microglobulin.
• Binding Groove: Formed by α1 + α2. It is closed at both ends.
• Peptide Size: Small! Only 8–10 amino acids fit.

Figure 1: The Endogenous Pathway. Viral proteins in the cytoplasm are chopped up by the Proteasome, pumped into the ER by TAP, loaded onto MHC-I, and sent to the surface to alert CD8+ T-cells.

---

3. MHC Class II (The Exogenous Pathway)

MHC-II is only found on Professional APCs (Dendritic cells, Macrophages, B-cells). Its job is to display things eaten from the outside (like bacteria) to tell CD4+ Helper T-cells: "Look what I found outside, we need to start a war!"

• Structure: Two equally sized chains: α chain and β chain.
• Binding Groove: Formed by α1 + β1. It is open at both ends (like a hotdog bun).
• Peptide Size: Larger! 13–25 amino acids fit.

The MHC-II Processing Pathway Highly Tested

1. Phagocytosis: Bacteria are eaten and destroyed inside a Lysosome, creating exogenous peptides. 2. ER Assembly: MHC-II is built in the ER. To prevent endogenous (viral) peptides from accidentally falling into the groove, the groove is plugged by the Invariant Chain (Ii). 3. Endosome: MHC-II travels to the endosome. Enzymes chew away most of the Invariant Chain, leaving only a tiny stump in the groove called CLIP. 4. HLA-DM: A specialized protein called HLA-DM rips CLIP out of the groove, allowing the bacterial peptide to finally bind! The complex moves to the surface to activate CD4+ cells.

---

4. Master Comparison: MHC I vs. MHC II

Feature	MHC Class I	MHC Class II
Human Genes (HLA)	A, B, C	DP, DQ, DR
Expression	ALL nucleated cells	APCs only (DCs, Macrophages, B-cells)
Recognized By	CD8+ Cytotoxic T-cells	CD4+ Helper T-cells
Antigen Source	Endogenous (Viral/Tumor inside cell)	Exogenous (Bacteria eaten from outside)
Peptide Size	8–10 amino acids (Closed ends)	13–25 amino acids (Open ends)
Key Transport/Helper Proteins	Proteasome, TAP transporter	Lysosome, Invariant Chain (Ii), CLIP, HLA-DM

Memory Trick: The Rule of 8

MHC I × CD8 = 8.
MHC II × CD4 = 8.

---

5. MHC Genetics (Polymorphism & Haplotypes)

The MHC genes are the most polymorphic (diverse) genes in the human genome. For example, there are over 7,000 different known alleles for HLA-B! This makes finding a perfect match for an organ transplant incredibly difficult.

• Polygenic: You have multiple different genes for MHC-I (A, B, and C).
• Codominant Expression: You inherit one complete set of genes (a Haplotype) from your mother, and one from your father. Both are fully expressed on your cells at the same time.

Clinical Importance

Certain HLA alleles are strongly linked to autoimmune diseases:

• HLA-B27: Ankylosing spondylitis.
• HLA-DR3/DR4: Type 1 Diabetes, Rheumatoid Arthritis.
• HLA-DQ2/DQ8: Celiac Disease.

---

6. Hybridoma Technology (Monoclonal Antibodies)

Invented by Köhler and Milstein (Nobel Prize 1984), this technique solves a massive problem: Normal B-cells make specific antibodies but die quickly in culture. Myeloma (cancer) cells live forever in culture, but don't make useful antibodies. The solution is to fuse them!

1. Immunize mouse with antigen → Harvest spleen (Extract B-cells).
2. Obtain HGPRT-negative Myeloma cells.
3. Fuse them using PEG (Polyethylene glycol).
4. You now have a mix of unfused B-cells, unfused Myeloma cells, and fused Hybridomas.
5. Plate the cells in HAT Selection Medium.

---

7. The Magic of HAT Medium

HAT stands for Hypoxanthine, Aminopterin, and Thymidine.

Why do only Hybridomas survive? Must Know

Cells have two ways to synthesize DNA nucleotides: The De novo pathway (from scratch) and the Salvage pathway (recycling).

1. Aminopterin completely poisons the De novo pathway. ALL cells are forced to use the Salvage pathway. 2. The Salvage pathway strictly requires the enzyme HGPRT. Unfused Myeloma Cells: We intentionally used a mutant strain that lacks HGPRT. Since the De novo pathway is blocked, and they can't use the salvage pathway, they die. Unfused B-cells: They have HGPRT and can survive the medium, but because they are normal cells, they reach senescence and die of old age within days. Hybridomas: They survive! They inherited the immortality from the myeloma cell, and they inherited the HGPRT enzyme from the B-cell!

---

8. High-Yield CSIR-NET / GATE Memory Tricks

Lock these facts in before your exam! 🚀

• 1. Rule of 8: CD8 pairs with MHC-I. CD4 pairs with MHC-II.
• 2. MHC-I Peptide Size: 8-10 amino acids (closed groove).
• 3. MHC-II Peptide Size: 13-25 amino acids (open groove).
• 4. TAP Transporter: Pumps endogenous peptides into the ER for MHC-I.
• 5. Invariant Chain (Ii): Plugs the MHC-II groove in the ER.
• 6. HLA-DM: The enzyme that rips the CLIP fragment out of MHC-II so the real antigen can bind.
• 7. PEG (Polyethylene glycol): Used to fuse B-cells and Myeloma cells.
• 8. Aminopterin: The drug in HAT medium that blocks the De novo nucleotide synthesis pathway.
• 9. HGPRT: The salvage pathway enzyme inherited from the B-cell that allows the Hybridoma to survive in HAT medium.
• 10. Suffixes: -omab (Mouse), -ximab (Chimeric), -zumab (Humanized), -umab (Fully human).

---

9. Fun & High-Yield Master Quiz!

CSIR NET & GATE Master Quiz

Let's test those analytical skills! These 10 questions match the exact logic of high-level life science examinations. You've got this!

1. During the assembly of MHC Class I molecules in the Endoplasmic Reticulum, which of the following proteins associates with the heavy α chain, despite NOT being encoded within the MHC gene locus on Chromosome 6?

A) TAP B) β2-microglobulin C) Invariant Chain D) HLA-DM

[Correct Answer: B] Masterful! The α chain is encoded by HLA-A, B, or C on chromosome 6. However, β2-microglobulin (the tiny structural partner) is encoded on chromosome 15. Without it, MHC-I cannot reach the cell surface!

2. A patient with a rare genetic defect lacks functional TAP transporters. Which immune process will be most directly and severely compromised?

A) Presentation of exogenous antigens on MHC-II to CD4+ T-cells. B) Presentation of endogenous viral peptides on MHC-I to CD8+ T-cells. C) Production of antibodies by B-cells. D) The function of the Invariant Chain.

[Correct Answer: B] Spot on! TAP (Transporter Associated with Antigen Processing) acts as a pump, moving peptides chopped up by the proteasome from the cytoplasm into the ER, where empty MHC-I molecules are waiting. Without TAP, MHC-I remains empty and degrades.

3. In the exogenous (MHC-II) processing pathway, what is the specific biological role of the non-classical MHC molecule, HLA-DM?

A) It forms the actual peptide-binding groove of MHC-II. B) It transports the MHC-II complex from the ER to the Golgi. C) It facilitates the removal of the CLIP fragment from the MHC-II groove within the endosome, allowing the high-affinity antigenic peptide to bind. D) It presents lipid antigens to T-cells.

[Correct Answer: C] Excellent! The Invariant Chain plugs the MHC-II groove in the ER. In the endosome, enzymes chew the chain down to a tiny stump called CLIP. HLA-DM physically rips CLIP out of the groove so the real bacterial peptide can load.

4. Which of the following statements correctly describes the structural constraints of the MHC-I and MHC-II peptide-binding grooves?

A) MHC-I has an open-ended groove accommodating large peptides (13-25 aa), while MHC-II has a closed-ended groove for small peptides (8-10 aa). B) Both accommodate peptides of any size. C) MHC-I has a closed-ended groove restricting peptides to 8-10 aa; MHC-II has an open-ended groove allowing larger peptides (13-25 aa) to hang out of the ends. D) MHC-I binds only carbohydrates, while MHC-II binds proteins.

[Correct Answer: C] Brilliant! Think of MHC-I like a tight pita bread (hot dog bun pinched at the ends) that only fits small 8-10 aa peptides. MHC-II is like an open hot dog bun where larger peptides (13-25 aa) can hang out over the edges!

5. In Hybridoma technology, why is it absolutely necessary that the Myeloma cells used for the fusion are mutants lacking the HGPRT enzyme?

A) To ensure they cannot produce their own antibodies. B) To ensure they will die in the HAT selection medium if they fail to fuse with a normal B-cell. C) To make them immortal. D) To increase the efficiency of PEG fusion.

[Correct Answer: B] You nailed it! The HAT medium uses Aminopterin to block the main (de novo) DNA synthesis pathway. Cells MUST use the salvage pathway (which requires HGPRT) to survive. By using an HGPRT- myeloma, we guarantee that any unfused myeloma cells will starve and die, leaving only the successful hybridomas!

6. You are analyzing the genome of a patient to determine their HLA haplotype for an organ transplant. How are the HLA alleles genetically expressed on the patient's cell surfaces?

A) Only the maternal alleles are expressed. B) Only the paternal alleles are expressed. C) They are expressed codominantly; both the maternal and paternal alleles are simultaneously expressed on the cell surface. D) Allelic exclusion ensures only one allele is expressed per cell.

[Correct Answer: C] Spot on! Unlike B-cells (which use allelic exclusion for antibodies), MHC genes are Codominant. You want maximum diversity to present as many different pathogens as possible, so your cells display both mom's and dad's HLA proteins simultaneously!

7. The therapeutic monoclonal antibody Trastuzumab is widely used in oncology. Based on its suffix (-zumab), what is the structural origin of this antibody?

A) It is a 100% murine (mouse) antibody. B) It is a chimeric antibody (mouse variable region + human constant region). C) It is a humanized antibody (only the CDR loops are mouse, the rest is human). D) It is a fully human antibody.

[Correct Answer: C] Perfect memory! -omab = fully mouse. -ximab = chimeric. -zumab = humanized. -umab = fully human. Humanizing the antibody prevents the patient's immune system from recognizing the drug as foreign and destroying it!

8. Which of the following cell types is entirely incapable of expressing MHC Class I molecules on its surface?

A) Dendritic cells B) Neurons C) Mature Erythrocytes (RBCs) D) Hepatocytes (Liver cells)

[Correct Answer: C] Excellent! MHC-I is expressed on all NUCLEATED cells. Since mature human red blood cells eject their nucleus during development, they do not express MHC-I. (This is why Plasmodium parasites hide inside RBCs during malaria!)

9. What is the fundamental concept of "MHC Restriction" discovered by Doherty and Zinkernagel?

A) T-cells can only recognize an antigen if it is presented by an MHC molecule of the same genetic background (self-MHC). B) MHC molecules restrict the entrance of viruses into the cell. C) MHC-II is restricted only to B-cells. D) T-cells can recognize free-floating antigens if they are bound to antibodies.

[Correct Answer: A] Masterful! The T-Cell Receptor (TCR) is highly specific. It doesn't just recognize the viral peptide; it recognizes the peptide AND the specific structure of your own personal MHC molecule holding it.

10. "Cross-presentation" is a unique immunological phenomenon crucial for generating a strong immune response against viral infections. What occurs during cross-presentation?

A) A virus jumps from an MHC-I molecule directly to an MHC-II molecule. B) B-cells present endogenous antigens on MHC-II. C) A specialized Dendritic cell phagocytoses an exogenous viral antigen and "crosses" the normal rules to present it on MHC-I, thereby activating naive CD8+ Cytotoxic T-cells. D) An antibody binds to two different antigens simultaneously.

[Correct Answer: C] Exactly! Normally, exogenous things go on MHC-II (to activate CD4s). But if a dendritic cell eats a dead, virus-infected cell, it NEEDS to activate the CD8+ assassins. It bends the rules, routing the exogenous viral protein into the endogenous MHC-I pathway!