Questions: Cancer Immunotherapy: CAR-T, Checkpoint Inhibitors, and Vaccines

Question 1

A tumor downregulates MHC-I expression on its surface to avoid recognition by cytotoxic T cells. Which immunotherapy approach would be LEAST impaired by this immune evasion strategy?

• A. PD-1 checkpoint inhibitors, because they restore T cell activity regardless of MHC expression
• B. Cancer vaccines, because they prime T cells to recognize tumor neoantigens presented by APCs
• C. CAR-T cell therapy, because CARs bind directly to tumor surface proteins without requiring MHC presentation
• D. CTLA-4 inhibitors, because CTLA-4 blockade enhances T cell priming in lymph nodes independently of tumor MHC

Answer: (C)

The key insight from the Core Idea and Explainer: CAR-T cells use a synthetic chimeric antigen receptor that binds directly to a target protein on the tumor cell surface — bypassing the MHC presentation machinery entirely. Normal cytotoxic T cells require their TCR to bind a peptide-MHC complex, so MHC-I downregulation is an effective evasion strategy against them, and against vaccines or checkpoint inhibitors that work by enhancing TCR-based recognition. CARs are specifically designed to circumvent this evasion mechanism. This is one of the primary advantages of CAR-T therapy over approaches dependent on natural T cell recognition.

Question 2

A patient with metastatic melanoma is treated with pembrolizumab (anti-PD-1). The treatment produces no clinical response. Which factor most likely explains the lack of benefit?

• A. The tumor has upregulated MHC-I, making T cell recognition too strong to be blocked by PD-1
• B. The tumor lacks pre-existing tumor-infiltrating T cells, so there are no T cells for the checkpoint inhibitor to release
• C. Pembrolizumab cannot cross the blood-tumor barrier in melanoma
• D. PD-1 inhibitors only work in blood cancers, not solid tumors

Answer: (B)

Checkpoint inhibitors 'release the brakes' on existing T cells, but they require that tumor-reactive T cells already be present and suppressed. If a tumor has not been recognized by the immune system at all — no T cell infiltration, no pre-existing anti-tumor immunity — there are no brakes to release. This is one reason checkpoint blockade benefits only ~30-40% of patients. Effective responders tend to have tumors with high mutational burden (generating more neoantigens), pre-existing T cell infiltration ('hot' tumors), and PD-L1 expression (confirming the checkpoint is actually suppressing T cells). 'Cold' tumors with no immune infiltrate are unlikely to respond.

Question 3

CAR-T cells can recognize and kill tumor cells that have lost MHC-I expression, because CARs bind directly to tumor surface proteins without requiring antigen presentation.

• T. True
• F. False

Answer: True

This is the defining feature of CAR-T therapy that distinguishes it from conventional T cell recognition. The chimeric antigen receptor contains an extracellular antibody fragment (scFv) that binds directly to a target protein on the tumor surface — analogous to how an antibody binds its antigen — linked to intracellular T cell signaling domains. There is no TCR-MHC interaction at any step. Since MHC downregulation is one of the most common tumor immune evasion strategies, this MHC-independence is a genuine therapeutic advantage, particularly for tumors that have evolved to avoid TCR-based detection.

Question 4

Checkpoint inhibitors such as pembrolizumab directly kill tumor cells by blocking PD-L1 on the tumor surface.

• T. True
• F. False

Answer: False

Checkpoint inhibitors do not directly kill tumor cells. They remove inhibitory signals from T cells. PD-1 inhibitors (like pembrolizumab) bind PD-1 on T cells, blocking its interaction with PD-L1 on tumor cells. This prevents the tumor from applying the brakes to tumor-reactive T cells, allowing those T cells to remain active and carry out cytotoxic killing. The drug enables the immune response — it does not itself kill tumors. This distinction matters clinically: if there are insufficient tumor-reactive T cells, releasing the brakes produces no therapeutic effect, which explains why many patients do not respond.

Question 5

Why do combination immunotherapy approaches (e.g., checkpoint inhibitor plus cancer vaccine, or CAR-T followed by checkpoint blockade) often outperform single-modality treatments?

Model answer: Each immunotherapy modality addresses a different bottleneck in the anti-tumor immune response. Checkpoint inhibitors release suppression of existing T cells but require pre-existing tumor-reactive T cells to work. Cancer vaccines prime or expand tumor-reactive T cells but may be suppressed once those T cells reach the tumor microenvironment. CAR-T cells are potent against hematological cancers but face immunosuppression in solid tumor microenvironments. Combining modalities addresses multiple bottlenecks simultaneously: a vaccine can generate the T cells that checkpoint inhibitor then unleashes; checkpoint blockade can prevent exhaustion of CAR-T cells in the tumor. Because resistance to one modality often operates through a pathway that a second modality addresses, combinations can be synergistic rather than merely additive.

The logic of combination therapy in oncology generally — and immunotherapy specifically — is that tumors evolve resistance to individual pressures. A tumor that escapes checkpoint blockade by antigen loss can still be targeted by a vaccine against a different antigen. A tumor that escapes CAR-T cells by downregulating the target antigen (e.g., CD19) might be vulnerable to checkpoint-released endogenous T cells targeting other antigens. Combinations reduce the probability that any single resistance mechanism defeats the entire treatment.