Class I and class II MHC molecules are membrane-bound glycoproteins that present antigenic peptides to T cells. Class I MHC molecules present intracellular peptides to CD8+ T cells and are expressed on all nucleated cells. They are composed of an alpha chain associated with beta-2 microglobulin. Class II MHC molecules present extracellular peptides to CD4+ T cells and are expressed mainly on antigen presenting cells. They are composed of alpha and beta chains. Viruses can induce malignant cell transformation by encoding oncoproteins that alter normal cell growth properties.

1. CANCER
IMMUNOLOGY

2. MHC Molecules

3. MHC Molecules and Genes
Class I and class II MHC molecules are membrane-bound glycoproteins that are
closely related in both structure and function.
Both class I and class II MHC molecules have been isolated and purified and the
three-dimensional structures of their extracellular domains have been determined
by x-ray crystallography.
Both types of membrane glycoproteins function as highly specialized antigen-
presenting molecules that form unusually stable complexes with antigenic
peptides, displaying them on the cell surface for recognition by T cells.
Class III MHC molecules are a group of unrelated proteins that do not share
structural similarity and common function with class I and II molecules.

5. TYPES OF MHC MOLECULES
MHC I MHC II MHC III

6. Class I MHC
Molecules
Class I MHC molecules contain a
45-kilodalton (kDa) α chain
associated noncovalently with a 12-
kDa β2-microglobulin molecule.
The α chain is a transmembrane
glycoprotein encoded by
polymorphic genes within the A, B,
and C regions of the human HLA
complex and within the K and D/L
regions of the mouse H-2 complex.
β2-Microglobulin is a protein
encoded by a highly conserved gene
located on a different chromosome.
Association of the α chain with β2-
microglobulin is required for
expression of class I molecules on
cell membranes.

7. The α chain is anchored in the plasma membrane by its hydrophobic transmembrane
segment and hydrophilic cytoplasmic tail.
Structural analyses have revealed that the α chain of class I MHC molecules is organized
into three external domains (α1, α2, and α3), each containing approximately 90 amino
acids.
A transmembrane domain of about 25 hydrophobic amino acids followed by a short
stretch of charged (hydrophilic) amino acids; and a cytoplasmic anchor segment of 30
amino acids.
The β2-microglobulin is similar in size and organization to the α3 domain; it does not
contain a transmembrane region and is noncovalently bound to the class I glycoprotein.
Sequence data reveal homology between the α3 domain,β2-microglobulin,and the
constant-region domains in immunoglobulins.
The enzyme papain cleaves the αchain just 13 residues proximal to its transmembrane
domain, releasing the extracellular portion of the molecule, consisting of α1, α2, α3, and
β2-microglobulin.

8. This peptide-binding cleft is located on the top surface of the class I MHC molecule, and
it is large enough to bind a peptide of 8–10 amino acids.
The great surprise in the x-ray crystallographic analysis of class I molecules was the
finding of small peptides in the cleft that had co crystallized with the protein.
The α3 domain and β2-microglobulin are organized into two pleated sheets each formed
by antiparallel β strands of amino acids known as the immunoglobulin fold.
sequence similarity with the immunoglobulin constant regions, class I MHC molecules
and β2-microglobulin are classified as members of the immunoglobulin superfamily.
In the absence of β2-microglobulin, the class I MHC β chain is not expressed on the cell
membrane.
This is illustrated by Daudi tumour cells, which are unable to synthesize β2-
microglobulin. These tumour cells produce class I MHC α chains, but do not express them
on the membrane.

9. CLASS II MHC
MOLECULE

10. Class II MHC molecules contain two different polypeptide chains, a 33-kDa α chain and
a 28-kDa β chain, which associate by noncovalent interactions.
Like class I α chains, class II MHC molecules are membrane-bound glycoproteins that
contain external domains, a transmembrane segment, and a cytoplasmic anchor segment.
Each chain in a class II molecule contains two external domains: α1 and α2 domains in
one chain and β1 and β2 domains in the other.
The membrane-proximal α2 and β2 domains, like the membrane-proximal α3/β2-
microglobulin domains of class I MHC molecules, bear sequence similarity to the
immunoglobulin-fold structure.
For this reason, class II MHC molecules also are classified in the immunoglobulin
superfamily.
The membrane-distal portion of a class II molecule is composed of the α1 and β1
domains and forms the antigen binding cleft for processed antigen.
X-ray crystallographic analysis reveals the similarity of class II and class I molecules,
strikingly apparent when the molecules are superimposed.

11. HUMAN
LEUKOCYTE
ANTIGEN

12. The major histocompatibility complex is a
collection of genes arrayed within a long
continuous stretch of DNA on chromosome 6 in
humans and on chromosome 17 in mice.
The MHC is referred to as the HLA complex in
humans and as the H-2 complex in mice.
Although the arrangement of genes is somewhat
different, in both cases the MHC genes are
organized into regions encoding three classes of
molecules.
Class I MHC genes encode glycoproteins
expressed on the surface of nearly all nucleated
cells; the major function of the class I gene
products is presentation of peptide antigens to TC
cells.

13. Class II MHC genes encode glycoproteins
expressed primarily on antigen-presenting
cells (macrophages, dendritic cells, and B
cells),where they present processed antigenic
peptides to TH cells.
Class III MHC genes encode, in addition to
other products, various secreted proteins that
have immune functions, including
components of the complement system and
molecules involved in inflammation.
Example: The expression of the class I HLA
- G molecules on cytotrophoblasts at the fetal-
maternal interface has been implicated in
protection of the fetus from being recognized
as foreign (this may occur when paternal
antigens begin to appear) and from being
rejected by maternal TC cells.

14. MALIGNANT
TRANSFORMATION OF
CELLS

15. Treatment of normal cultured cells with chemical carcinogens, irradiation, and
certain viruses can alter their morphology and growth properties.
Transformation makes the cells able produce tumours when they are injected into
animals.
Such cells undergo malignant transformation, and they often exhibit properties in
vitro similar those of cancer cells.
EXAMPLE: They have decreased requirements for growth factors & serum, are
no longer anchorage dependent, & grow in a density-independent fashion.
Various chemical agents (e.g., DNA-alkylating reagents) and physical agents (e.g.,
ultraviolet light and ionizing radiation) that cause mutations have been shown to
induce transformation.

17. • Induction of malignant transformation with chemical or physical carcinogens
appears to involve multiple steps and at least two distinct phases:
PROMOTIONINITIATION

18. Initiation involves changes in the genome but does not, in itself, lead to malignant transformation.
After initiation, promoters stimulate cell division and lead to malignant transformation.
The importance of mutagenesis in the induction of cancer is illustrated by diseases such as
xeroderma pigmentosum.
This rare disorder is caused by a defect in the gene that encodes a DNA-repair enzyme called UV-
specific endonuclease.
 Individuals with this disease are unable to repair UV-induced mutations and consequently develop
skin cancers.
A number of DNA and RNA viruses have been shown to induce malignant transformation. Two of
the best-studied are SV40 and polyoma.
In both cases the viral genomes, which integrate randomly into the host chromosomal DNA, include
several genes that are expressed early in the course of viral replication.
SV40 encodes two early proteins called large T and little T, and polyoma encodes three early
proteins called large T, middle T, and little T.
Each of these proteins plays a role in the malignant transformation of virus-infected cells.

19. Human cancers with well-established links to viral
infection include:
• Adult T – cell leukaemia/lymphoma, which occurs in a small
percentage of persons infected with human T-cell leukaemia
virus-1 (HTLV-1)
• Kaposi’s sarcoma, which is linked to human herpesvirus-8
(HHV-8) and usually occurs in those also infected with HIV-1
• Cervical carcinoma, which is linked to infection by one of
several serotypes of Human Papilloma virus (HPV)
• Liver carcinoma that follows infection with Hepatitis B virus
(HBV)
• Epstein – Barr virus (EBV) infection, which is linked to
Burkitt’s lymphoma in African population & Nasopharyngeal
carcinoma mainly in Asian populations.

21. Most RNA viruses replicate in the cytosol and do not induce malignant transformation.
The exceptions are retroviruses, which transcribe their RNA into DNA by means of a
reverse-transcriptase enzyme and then integrate the transcript into the host’s DNA.
This process is similar in the cytopathic retroviruses such as HIV-1 and HIV-2 and in the
transforming retroviruses, which induce changes in the host cell that lead to malignant
transformation.
In some cases, retrovirus-induced transformation is related to the presence of oncogenes,
or “cancer genes” carried by the retrovirus.
One of the best-studied transforming retroviruses is the Rous sarcoma virus.
This virus carries an oncogene called v-src, which encodes a 60-kDa protein kinase (v-
Src) that catalyses the addition of phosphate to tyrosine residues on proteins.
The first evidence that oncogenes alone could induce malignant transformation came from
studies of the v-src oncogene from Rous sarcoma virus.
 When this oncogene was cloned and transfected into normal cells in culture, the cells
underwent malignant transformation.

22. Oncogenes and Cancer Induction

23. In 1971, Howard Temin suggested that oncogenes might not be unique to transforming viruses but
might also be found in normal cells.
He proposed that a virus might acquire oncogenes from the genome of an infected cell. He called
these cellular genes proto-oncogenes, or cellular oncogenes (c-onc), to distinguish them from
their viral counterparts (v-onc).

24. In the mid-1970s, J. M. Bishop and H. E. Varmus identified a DNA sequence in
normal chicken cells that is homologous to v-src from Rous sarcoma virus.
 This cellular oncogene was designated c-src.
Since these early discoveries, numerous cellular oncogenes have been identified.
Sequence comparisons of viral and cellular oncogenes reveal that they are highly
conserved in evolution.
Although most cellular oncogenes consist of a series of exons and introns, their
viral counterparts consist of uninterrupted coding sequences, suggesting that the
virus might have acquired the oncogene through an intermediate RNA transcript
from which the intron sequences had been removed during RNA processing.
The actual coding sequences of viral oncogenes and the corresponding proto-
oncogenes exhibit a high degree of homology.

25. In some cases, a single point mutation is all that distinguishes a viral oncogene
from the corresponding proto-oncogene.
It has now become apparent that most, if not all, oncogenes (both viral and
cellular) are derived from cellular genes that encode various growth-controlling
proteins.
In addition, the proteins encoded by a particular oncogene and its corresponding
proto-oncogene appear to have very similar functions.
The conversion of a proto-oncogene into an oncogene appears in many cases to
accompany a change in the level of expression of a normal growth-controlling
protein.

26. REFERENCES
1.Kuby, A Textbook of Immunology(e-book),
Sixth edition, Page no:501 – 503, 166 – 168.