Innate Immune Cells and Phagocytic Mechanisms

The Intersection of Innate and Adaptive Immunity

• The cell that sits at the center of the immune system represents the bridge where the innate immune system stops and the adaptive immune system begins, provided additional antigen is presented.
• Innate immune cells are numerous and diverse; they are categorized primarily by two methods: their function and the markers that make a cell recognizable.
• One characteristic that makes a cell biological "delicious" or a target for consumption is the state of being dead.
• Death markers are critical for identification; for example, Phosphatidylserine is a molecule expressed by dead cells that signals a phagocyte to begin consumption.

Phagocytosis and the Process of Phagosome Maturation

• When a phagocyte detects a particle that has been opsonized, it undergoes significant changes in its molecular cytoskeleton.
• The phagocyte extends itself to enclose those particles within a specialized internal compartment called a phagosome.
• Phagosome maturation is the internal process of preparing the contents for destruction, similar to the digestive process in a stomach where acid is added to ensure whatever has been consumed is "made dead."
• The lysosome is a highly dangerous cellular compartment containing digestive enzymes and acid; the fusion of a phagosome with a lysosome is essential to prevent survival of pathogens.
• Pathogens such as TB (Tuberculosis) are noted for their ability to live within a phagosome by specifically preventing this fusion with the lysosome, thereby evading destruction.

The Respiratory Burst and Chemical Killing Mechanisms

• The respiratory burst is a metabolic process that produces free radicals to damage phagocytosed material. It begins with oxygen.
• Free radicals are molecularly unstable and chemically dangerous because they scavenge electrons, setting off chemical chain reactions through oxidation and reduction.
• The sequence of chemical production in the respiratory burst includes:
  • Oxygen ($O_2$) is converted into superoxide ($O_2^{\cdot-}$) by the enzyme NADPH oxidase. Superoxide has three electrons where oxygen typically has two.
  • Hydrogen peroxide ($H_2O_2$) is then formed from superoxide by the enzyme superoxide dismutase.
  • Hydrogen peroxide is chemically equivalent to bleach used for teeth or hair whitening.
• To increase toxicity, hydrogen peroxide is broken down and combined with halide ions, such as chloride or bromide, to form highly toxic substances:
  • Hypochlorite ($ClO^-$)
  • Hypobromite ($BrO^-$)
• Hypochlorite is the active ingredient in strong toilet cleaners, such as Jerk, which is used to kill everything in its path due to its extreme toxicity.
• Once the phagosome has fused with the lysosome, it contains a lethal combination of acid, digestive enzymes, and free radicals that oxidize the pathogen. This state is known as a phagolysosome.

Neutrophils and Neutrophil Extracellular Traps (NETs)

• Neutrophils are the first line of attack in the immune system. They often act independently of the adaptive immune response, attempting to solve problems without involving T cells.
• A patient lacks neutrophils is considered a "dead patient walking" due to their critical role in early defense.
• Neutrophils are non-professional antigen-presenting cells; they phagocytose pathogens but do not "snitch" by presenting them to T cells.
• When neutrophils cannot handle a problem through phagocytosis alone, they undergo a process where they extrude their cellular DNA, effectively "vomiting" their nuclear material to create a NET (Neutrophil Extracellular Trap).
• Neutrophil Extracellular Traps (NETs):
  • Consist of sticky DNA, associated proteins, and toxic histones.
  • Neutrophils attach their granules, including lysosomal granules, to these nets, creating a "poisoned mosquito net" in the environment.
  • Bacteria become trapped in the sticky DNA and are killed by the associated toxins and histones.
  • This process is suicidal; a neutrophil cannot survive without its nuclear material and dies after extruding its NET.
• Pus consists of dead and dying neutrophils, along with the bacteria that provoked their response.
• Neutrophils are characterized by a multi-lobed nucleus and have a lifespan of approximately $3\,\text{years}$.

Monocytes and Macrophages: The Reticuloendothelial System

• Monocytes have a horse-shoe-shaped nucleus. They circulate in the blood briefly before migrating into tissues to become macrophages.
• Macrophages, meaning "big eaters," are long-lived and seeded throughout the body during infancy to form the reticuloendothelial system:
  • Kupffer cells in the liver.
  • Alveolar macrophages in the lung.
  • Microglia or glial cells in the brain.
• Functions of Macrophages:
  • Constant surveillance and control of tissues.
  • Recognition of trouble through pattern recognition receptors.
  • Professional antigen presentation using MHC Class II molecules ($\text{MHC-II}$).
• When macrophages cannot kill what they eat, such as non-living materials (nickel, asbestos) or resistant bacteria like TB, they may undergo specialized changes:
  • They can "vomit" toxic molecules into surrounding tissue, which may cause bystander necrosis.
  • They can fuse together to form giant multinucleated cells called syncytia to collaborate on solving the problem.
• Adipocytes (fat cells) are also related to the macrophage lineage.

Dendritic Cells and Antigen Presentation

• Myelodendritic cells make up a small proportion of circulating cells and are identified by their fluffy, dendritic projections.
• These projections increase the surface area for antigen presentation, allowing for an immunological synapse with multiple copies of $\text{MHC-II}$.
• Dendritic cells tell $CD4^+$ T cells whether to differentiate into T helper 1 ($T_H1$) or T helper 2 ($T_H2$) cells.
  • $T_H1$ cells provide help for cellular immunity.
  • $T_H2$ cells help B cells produce antibodies.

Innate Lymphoid and Natural Killer (NK) Cells

• Innate lymphoid cells originate from the common lymphoid progenitor in the bone marrow but lack the specific BCR (B-cell receptor) or TCR (T-cell receptor) that defines the adaptive immune system.
• Because their receptors do not change or rearrange, they lack memory and the fine-tuned specificity of adaptive lymphocytes.
• Natural Killer (NK) cells:
  • These are lymphocytes that are neither $CD4^+$ T cells, $CD8^+$ T cells, nor B cells.
  • They monitor cells for the downregulation of self-antigens.
  • They operate on the "Missing Self Hypothesis": if a virus or tumor downregulates MHC Class I ($\text{MHC-I}$) to hide from cytotoxic T cells, the NK cell detects the absence and kills the cell.
  • NK cells have activating switches and inhibitory switches; the strongest inhibitory signal is the presence of $\text{MHC-I}$.
  • They kill by pressing the Fas switch (a death receptor) and by injecting poisons or punching holes in the cell wall (perforin/granzymes).
  • NK cells also perform Antibody-Dependent Cellular Cytotoxicity (ADCC).
• Natural Killer T (NKT) Cells:
  • These recognize lipids and carbohydrate antigens rather than peptides.
  • Peptide recognition is highly specific because changing one amino acid in a string changes the whole protein.
  • Lipids are just chains of repeated molecules, making them harder to distinguish; NKT cells recognize glycosylated lipids, peptidoglycans, and mannoses not found in humans.

Granulocytes: Basophils and Eosinophils

• The immune system employs specific strategies for large threats like tapeworms that cannot be phagocytosed.
• If a pathogen is too large to eat, the immune system makes the environment uninhabitable by turning off "lights and music" (removing resources) and flooding the area with toxins.
• Basophils and Mast Cells:
  • Responsible for the immediate response to allergens and parasites.
  • They secrete histamine, which makes the skin or tissue an unpleasant environment (e.g., mosquito bites becoming red, itchy, and hot to prevent further feeding).
  • Histamine serves as a natural insect repellent.
• Eosinophils:
  • These cells stain a distinct pink color.
  • They arrive after basophils and mast cells to deal with the late effects of activation.
  • They are highly destructive; in untreated IgE-mediated asthma, eosinophils cause tissue remodeling in the lungs by destroying local tissue.

Questions & Discussion

• Question: Why is it better to be killed by an NK cell than a cytotoxic T cell if you are a virus?
• Answer: It is actually worse for the virus long-term if it is killed by a T cell because T cells have memory and will hunt that specific virus down in the future. Choosing an NK cell is like choosing to be a bystander that just happens to get hurt rather than having an organization infiltrated and picked off one by one by specific hunters.
• Question: Why do viruses never infect mature red blood cells?
• Answer: Red blood cells are essentially "dead" in terms of cellular machinery. They lack a nucleus, a kitchen (organelles), and a bedroom; they are just four walls. A virus cannot grow a family or reproduce in a cell that has no nucleus to hijack.
• Question: What is an example of a natural antibody?
• Answer: ABO blood group antibodies are natural antibodies against different vectors; they are typically of the IgM class.