PHAL311- lecture 20

What is immune suppression?

Immune suppression occurs when chemicals or drugs reduce immune function by killing immune cells or altering their differentiation, surface markers, or signalling. This increases the risk of infection, cancer, tissue damage, and poor vaccine responses.

How does high-dose methotrexate act in chemotherapy?

High-dose methotrexate acts as a cytotoxic chemotherapeutic by entering cells via folate receptors, becoming activated by glutamate addition, and inhibiting nucleotide synthesis enzymes. This blocks DNA synthesis in rapidly dividing cells, reducing thymidine, adenine, and guanine production, leading to death of cancer and immune cells.

What are the effects of high-dose methotrexate on immune organs?

High-dose methotrexate causes thymic atrophy (reduced T cell production and egress) and bone marrow suppression, leading to reduced leukocytes, RBCs, and platelets, resulting in anemia and impaired clotting.

How does low-dose methotrexate suppress the immune system?

Low-dose methotrexate inhibits transformylase activity, leading to accumulation of AMP and adenosine. Extracellular adenosine activates A2A GPCRs on immune cells, increasing cAMP/MAPK signalling and suppressing immune activation while promoting regulatory immune responses.

What are the immunological effects of adenosine signalling in methotrexate treatment?

Adenosine signalling suppresses dendritic cells, macrophages, NK cells, T cells, and B cells while promoting regulatory T cells. It increases IL-10 and decreases IL-12, IL-6, and IL-8, shifting immunity toward an anti-inflammatory state.

What are the clinical consequences of methotrexate-induced immune modulation?

Methotrexate increases UV sensitivity and impaired wound healing, increases infection risk, reduces vaccine responses, and increases cancer risk due to reduced cytotoxic immune surveillance.

What are PAHs and how do they affect immunity?

PAHs (e.g. TCDD, PCBs, and tryptophan metabolites like I3C and FICZ) activate the aryl hydrocarbon receptor (AhR) in epithelial cells, triggering transcriptional changes that suppress immune function.

What happens when AhR is activated by PAHs?

AhR moves from the cytoplasm to the nucleus and induces CYP450 enzymes, alters immune cell differentiation, and increases its own expression, amplifying signalling and promoting detoxification and immune reprogramming.

How does AhR activation alter immune signalling pathways?

AhR inhibits STAT1, NF-κB, and HIF-1α while activating Src, FOXP3, and RORγt, shifting immunity from pro-inflammatory responses toward regulatory and reparative pathways.

How does AhR activation change cytokine production?

AhR increases IL-10, IL-17, IL-21, and IL-22 while decreasing IFN-γ and IL-6, leading to reduced inflammation, reduced antigen presentation, and increased regulatory T cell activity and tissue repair.

What are the systemic effects of PAH exposure on immune organs?

PAHs cause thymic atrophy via reduced T cell proliferation, disrupt bone marrow output, cause skin disorders (e.g. acne-like lesions), increase infection susceptibility, reduce vaccine responses, and increase cancer risk (lung, skin, GI, myeloma).

How are in vivo studies used to assess immune suppression?

In vivo testing includes measuring immune organ size/histology (thymus, spleen, lymph nodes), blood cell counts (RBCs, WBCs, platelets), and functional immune responses such as antibody production in TDAR assays or response to infection.

What is the TDAR assay?

The T-cell dependent antibody response (TDAR) measures antibody production after immunisation with a known antigen. in the presence of an immunosuppressive drug, reduced antibody levels indicate impaired T cell-dependent immune function.

What is an in vivo immune challenge study?

Animals are exposed to an infectious agent (e.g. influenza) while receiving a test compound. Immune response strength is measured to assess whether the immune system can still effectively respond.

What are PBMC assays used for?

Peripheral blood mononuclear cells are isolated from blood and stimulated in vitro to assess whether drugs suppress immune activation, particularly dendritic cell and T cell responses.

What is a mixed lymphocyte reaction (MLR)?

MLR measures T cell activation when immune cells from different donors are cultured together. Immunosuppressive drugs reduce T cell proliferation and activation.

What do proliferation assays measure?

They measure immune cell division after stimulation using methods like BrdU incorporation, CellTrace dye dilution, or radioactive thymidine uptake. Reduced proliferation indicates immune suppression.

What are ELISA and multiplex assays used for in immune suppression studies?

They measure cytokines and antibodies released by immune cells. Reduced cytokine or antibody levels indicate suppressed immune function.