Questions: Cross-Presentation of Exogenous Antigens

Question 1

A subunit vaccine contains a viral protein (not the whole virus) injected into muscle. The vaccine generates a CD8+ cytotoxic T cell response. Which process best explains how CD8+ T cells were activated by an exogenous protein never synthesized inside the priming dendritic cell?

• A. The viral protein entered the dendritic cell's cytoplasm through the plasma membrane and was processed by proteasomes directly
• B. Cross-presentation — dendritic cells internalized the exogenous protein and routed it into the MHC class I pathway
• C. The viral protein was synthesized inside dendritic cells after reverse transcription from contaminating viral RNA
• D. CD4+ T cells presented the antigen on their own MHC-I molecules after receiving it from dendritic cells

Answer: (B)

The vaccine contains exogenous protein — not made inside the dendritic cell. The standard rule routes exogenous protein into endosomes → MHC-II → CD4+ T cells. But CD8+ activation requires MHC-I presentation, which means the antigen must have entered the cytosolic processing pathway. Cross-presentation is the answer: specialized dendritic cells internalize exogenous antigens and divert them to proteasomal degradation → TAP → ER → MHC-I loading. Option A is not a recognized mechanism — exogenous proteins don't diffuse through the plasma membrane. Option C is wrong because subunit vaccines contain protein, not nucleic acid. Option D is wrong — CD4+ T cells use MHC-II, not MHC-I.

Question 2

The cytosolic pathway of cross-presentation involves which sequence of events?

• A. Exogenous protein → endosome → MHC-II loading in the endosome → surface display for CD4+ T cells
• B. Exogenous protein → phagosome → translocation to cytoplasm → proteasome → TAP → ER → MHC-I loading → CD8+ T cell activation
• C. Exogenous protein → cytoplasm entry via plasma membrane → proteasome → MHC-I loading at the cell surface
• D. Exogenous protein → phagosome → lysosomal degradation → MHC-I loading in lysosomes → surface display

Answer: (B)

The cytosolic pathway routes exogenous antigen through the standard MHC-I loading machinery after escape from the endosomal compartment. The protein is first internalized into a phagosome (not lysosomes — option D conflates these), then translocated across the phagosomal membrane into the cytoplasm (possibly via Sec61), where proteasomes degrade it into peptides, TAP transports them into the ER, and MHC-I loads them for surface display to CD8+ T cells. Option A is the standard MHC-II pathway for exogenous antigens. Option C skips the critical endosomal internalization step — cross-presentation still begins with endocytosis.

Question 3

Most antigen-presenting cells can perform cross-presentation with equal efficiency, making dendritic cell subset specialization irrelevant to this process.

• T. True
• F. False

Answer: False

This is directly contradicted by the Common Misconceptions section. Cross-presentation is performed most efficiently by specialized dendritic cell subsets — in humans, particularly BDCA-3+ (CD141+) conventional dendritic cells; in mice, the equivalent CD8α+ DC subset. These cells have specialized intracellular machinery that promotes phagosomal escape into the cytoplasm and efficient MHC-I loading. Other cells (macrophages, B cells, other DC subsets) can cross-present under some conditions but with much lower efficiency. This specialization is central to understanding which cells prime naive CD8+ T cells in vivo.

Question 4

Cross-presentation is necessary because most pathogens do not directly infect dendritic cells, meaning CD8+ T cell responses against these pathogens cannot be initiated through the standard MHC-I endogenous pathway in DCs.

• T. True
• F. False

Answer: True

This is precisely the biological rationale for cross-presentation. The standard MHC-I pathway presents peptides from proteins synthesized *inside* the presenting cell. If a virus doesn't infect dendritic cells, those cells have no viral proteins in their cytoplasm and cannot activate CD8+ T cells via the standard pathway. Cross-presentation solves this: DCs capture debris (apoptotic fragments, viral proteins) from infected cells elsewhere in the body and present these exogenous antigens on MHC-I to prime naive CD8+ T cells — even without being infected themselves.

Question 5

Why is cross-presentation described as 'breaking the rule' of antigen processing, and what is the immunological significance of this rule violation?

Model answer: The standard rule is: exogenous antigens → endosomes → MHC class II → CD4+ T cells; endogenous antigens → proteasomes → MHC class I → CD8+ T cells. Cross-presentation breaks this by routing exogenous proteins into the MHC-I pathway. The significance is profound: without this, the immune system could not mount cytotoxic T cell responses against pathogens that don't infect DCs — which is most pathogens. Cross-presentation allows DCs to act as sentinels that sample infected tissue without being infected themselves, then prime the cytotoxic response needed to kill infected cells throughout the body.

The logic of MHC-I vs MHC-II makes evolutionary sense: MHC-I signals 'something is wrong inside this cell; kill it,' while MHC-II signals 'I've captured something from outside; help coordinate the response.' Cross-presentation bridges the gap — it allows the 'something is wrong in surrounding tissue' signal to still activate CD8+ killing machinery. This is also why killed and subunit vaccines can generate cytotoxic T cell responses despite containing no live virus.