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PA6.4-6 | Tumour Effects, Immunology & Laboratory Diagnosis — SDL Guide (Part 2)

Tumour Immunology — Antigens and Surveillance

⚑ AI image — pending faculty review (auto-QA score 6/10; best of 3 attempts)

Diagram showing immune surveillance of nascent tumour cells, tumour-specific and tumour-associated antigens, antigen presentation to CD8 T cells, and clinical evidence from immunosuppression and TIL prognosis. — **Panel A:** Immune surveillance overview showing nascent tumour cell, MHC class I, tumour antigen, CD8+ cytotoxic T lymphocyte, NK cell, macrophage, dendritic cell, perforin, granzymes, and apoptotic tumour fragments.. **Panel B:** Comparison of TSA and TAA: tumour-specific antigen present only on tumour cells, neo-antigen from somatic mutation, tumour-associated antigen over-expressed or aberrantly expressed on tumour cells but also present on normal cells.. **Panel C:** Antigen recognition pathway showing tumour antigen release, dendritic cell uptake, antigen presentation in lymph node, CD8+ T-cell activation, and cytotoxic killing of tumour cell.. **Panel D:** Clinical evidence showing increased lymphoma, Kaposi sarcoma, and HPV-driven tumour risk in immunosuppressed patients, plus tumour-infiltrating lymphocytes correlating with better prognosis..

The immune system continuously monitors for and destroys nascent tumour cells — a process called immune surveillance (proposed by Burnet and Thomas). Clinical evidence:

Immunosuppressed patients (organ transplants, HIV/AIDS) have dramatically higher rates of certain cancers (lymphomas, Kaposi sarcoma, HPV-driven tumours).
Tumour-infiltrating lymphocytes (TILs) correlate with better prognosis in many cancers.

Tumour antigens — molecules recognised as non-self by immune cells — fall into two broad classes:

Class	Full name	Basis	Examples
TSA	Tumour-specific antigen	Present ONLY on tumour cells	Neo-antigens from somatic mutations (unique to each tumour)
TAA	Tumour-associated antigen	Over-expressed or aberrantly expressed on tumour; also found on normal cells	AFP (normally fetal), PSA, CEA, HER2

Effector mechanisms of anti-tumour immunity:

Cytotoxic T lymphocytes (CTLs): CD8+ T cells recognise tumour peptides on MHC class I → perforin/granzyme-mediated killing. The primary adaptive mechanism.
NK cells: Kill tumour cells that have down-regulated MHC I (a common evasion strategy — NK cells are activated by 'missing self').
Antibody-dependent cellular cytotoxicity (ADCC): Tumour-specific antibodies opsonise tumour cells → Fc-receptor-mediated killing by NK cells and macrophages.
Macrophages (M1): Activated macrophages release ROS and NO, directly cytotoxic.

Immune Evasion by Tumours

A multi-panel diagram showing how tumours evade immune surveillance through antigen loss, immunosuppressive microenvironment, PD-L1 checkpoint signalling, and how immunotherapy restores anti-tumour T-cell activity. — **Panel A:** Overview of tumour cells escaping immune surveillance by cytotoxic T lymphocytes, with surviving tumour clones highlighted.. **Panel B:** Antigen loss and MHC I down-regulation preventing CD8+ CTL recognition through the T-cell receptor.. **Panel C:** Immunosuppressive tumour microenvironment containing TGF-β, IL-10, Tregs, suppressed effector T cells, and M2 macrophages.. **Panel D:** PD-L1 on tumour cells binding PD-1 on T cells, producing T-cell exhaustion or anergy.. **Panel E:** Checkpoint inhibitors blocking PD-1 or CTLA-4 and CAR-T cells targeting CD19-positive B-cell malignancy..

Tumours that succeed clinically have escaped immune surveillance. Key evasion mechanisms:

Antigen loss / down-regulation: Tumour cells with high immunogenicity are selectively killed (immunoediting); surviving clones have lost or down-regulated MHC I or tumour antigens — so CTLs cannot recognise them.

Immunosuppressive microenvironment: Tumours secrete TGF-β and IL-10, which suppress T-cell activation and convert effector T cells into regulatory T cells (Tregs). The tumour micro-environment (TME) is rich in Tregs and M2 macrophages (tumour-promoting).

Checkpoint exploitation — PD-L1: Many tumours up-regulate PD-L1 (Programmed Death Ligand 1) on their surface. PD-L1 binds PD-1 on T cells → T-cell exhaustion/anergy. This is the most clinically important evasion mechanism because it is directly targetable.

Basis of immunotherapy:

Checkpoint inhibitors: Monoclonal antibodies that block PD-1 (pembrolizumab, nivolumab) or CTLA-4 (ipilimumab) → release T-cell brake → restore anti-tumour killing. Used in melanoma, NSCLC, bladder cancer, and others.
CAR-T cell therapy: Patient's T cells are genetically engineered ex vivo to express a chimeric antigen receptor (CAR) targeting a tumour antigen (e.g., CD19 in B-cell lymphoma/ALL). Highly effective in select haematologic malignancies.

SELF-CHECK

A tumour up-regulates PD-L1 on its surface. What is the net immunological effect on tumour-reactive T cells?

A. Enhanced CTL proliferation via co-stimulation

B. T-cell exhaustion and anergy via PD-1 signalling

C. Increased NK cell recruitment and ADCC

D. Upregulation of MHC class I presentation

Reveal Answer

Answer: B. T-cell exhaustion and anergy via PD-1 signalling

PD-L1 on the tumour surface binds PD-1 on T cells, delivering an inhibitory signal that causes T-cell exhaustion and functional anergy. This is the mechanistic basis for using PD-1/PD-L1 checkpoint inhibitors (pembrolizumab, nivolumab) to restore anti-tumour immunity.

Laboratory Diagnosis — Histopathology and Cytology

A four-panel medical diagram compares biopsy, FNAC, and frozen section methods used for definitive laboratory diagnosis of neoplasia. — **Panel A:** Overview showing suspected neoplastic mass, tissue diagnosis requirement, lesion accessibility, required material, urgency, and arrows to excisional biopsy, incisional biopsy, core needle biopsy, FNAC, and frozen section.. **Panel B:** Biopsy comparison showing entire lesion removal in excisional biopsy, wedge sampling in incisional biopsy, hollow-needle tissue core in core needle biopsy, and preserved tissue architecture with tumour nests, stroma, and glands.. **Panel C:** FNAC cytology showing 23-25G needle aspiration, cytology smear slide, malignant cells, enlarged hyperchromatic nuclei, high nuclear-to-cytoplasmic ratio, and absence of tissue architecture.. **Panel D:** Frozen section workflow showing fresh intraoperative tissue, snap-freezing, cryostat at -20 degrees Celsius, thin sectioning, H&E staining, microscope review, and rapid report in about 20 minutes..

Definitive cancer diagnosis requires tissue. The choice of sampling technique depends on lesion accessibility, required material, and urgency.

Biopsy types:

Type	Description	Best for
Excisional biopsy	Entire lesion removed	Small, accessible lesions (skin, lymph node)
Incisional biopsy	Wedge from large lesion	Large soft-tissue tumours; when excision not feasible
Core needle biopsy (Tru-Cut)	Hollow-needle core of tissue	Breast, prostate, liver, kidney — preserves architecture
FNAC (Fine Needle Aspiration Cytology)	23–25G needle, cytological smear	Rapid diagnosis (thyroid, breast, lymph node, salivary gland); cytology only, no architecture

Frozen section: Intraoperative technique — tissue snap-frozen, cut at −20°C, H&E stained in ~20 min. Used to:
1. Confirm malignancy intraoperatively (dictates extent of resection).
2. Assess surgical margins (clear vs involved).
3. Identify lymph node involvement.

Limitation: ice crystal artefact reduces cytological detail; not for all specimen types.

Cytology (non-biopsy):
• Exfoliative cytology: Cervical Pap smear, sputum, urine, CSF, pleural/peritoneal fluid.
• Imprint cytology: Touch preparation of cut biopsy surface.

Routine histopathology (H&E) identifies tissue architecture + cellular features (pleomorphism, mitoses, necrosis, invasion) — the gold standard for diagnosis and grading.