Microglia: Structure and Function in CNS Inflammation

The innate immune system employs multiple cellular defense mechanisms including Toll-Like Receptors (TLRs) that recognize pathogen-associated molecular patterns (PAMPs) through specific receptor pairs (e.g., TLR1/2 for gram-positive bacteria, TLR4 for gram-negative LPS, TLR3/7/8/9 for viral nucleic acids), triggering inflammatory cytokine release (TNF-alpha, IL-1, IL-6, IL-8, IL-12) that activates other immune cells and induces systemic responses like fever; professional phagocytes (neutrophils, macrophages, dendritic cells) eliminate pathogens through receptor-mediated binding, engulfment, and digestion via oxygen-dependent (reactive oxygen species) and oxygen-independent (lysosomal enzymes, antimicrobial peptides) mechanisms, with antigen-presenting cells presenting processed antigens on MHC class II molecules to initiate adaptive immunity; inflammation manifests through four cardinal signs (swelling, pain, heat, redness) driven by vasodilation, increased vascular permeability, and leukocyte recruitment, representing the body's coordinated nonspecific defense against infection.

The innate immune system provides the first line of defense through non-specific mechanisms. Macrophages engulf pathogens through phagocytosis, fuse with lysosomes for digestion, and expel waste via exocytosis. When pathogens are not eliminated, inflammatory responses recruit additional immune cells. Macrophages process antigens by breaking down pathogens and presenting fragments on MHC class II proteins to helper T cells. Helper T cells become activated and release cytokines that coordinate immune responses. This system provides immediate but non-specific protection against all pathogens.

The innate immune response involves specialized cells including granulocytes (mast cells, neutrophils, eosinophils, basophils), professional phagocytes (dendritic cells, macrophages), and natural killer cells, which release inflammatory mediators like histamine, cytokines, and chemokines to cause vasodilation, increased vascular permeability, and leukocyte recruitment through processes such as margination, adhesion, diapedesis, and chemotaxis, ultimately leading to phagocytosis and the creation of antigen-presenting cells that bridge innate and adaptive immunity.

Innate immunity provides the first line of defense through pattern recognition receptors (PRRs) that detect pathogen-associated molecular patterns (PAMPs) - conserved microbial components that cannot be easily mutated away. Toll-like receptors (TLRs) are key PRRs: TLR4 recognizes bacterial lipopolysaccharide, while TLR5 recognizes bacterial flagellin. Beneficial bacteria remain in mucus, but pathogenic bacteria use flagella to penetrate epithelial barriers, triggering immune responses. The inflammatory response follows a coordinated sequence: barrier breach, phagocyte recruitment via PRRs, cytokine secretion causing vasodilation (warmth/redness), leukocyte rolling and migration through endothelial selectins, and increased vascular permeability allowing antibodies and complement entry. This produces the four cardinal signs: calor, rubor, tumor, and dolor. After pathogen clearance, dendritic cells migrate to lymph nodes where they present antigens to T cells, initiating adaptive immunity.

Professional phagocytes in innate immunity include neutrophils, monocytes/macrophages, and dendritic cells. Neutrophils perform rapid phagocytosis and respiratory burst but cannot present antigens. Monocytes circulate and differentiate into tissue macrophages or dendritic cells upon migration. Crucially, macrophages and dendritic cells can process bacterial antigens and present them on their surfaces to activate adaptive immunity. This antigen presentation capability distinguishes them from neutrophils and represents the critical bridge between innate and adaptive immune responses. Natural killer cells also participate in innate defense against virus-infected cells.

The brain is considered an immunologically privileged organ because the immune system cannot recognize it as 'self.' This is due to the high concentration of specific proteins in the brain that the immune system cannot learn to recognize. The blood-brain barrier isolates the nervous system from the body's immune system, preventing immune cells and pathogens from freely entering the brain. When this barrier is compromised (such as during viral infections), immune cells can enter the brain, causing widespread inflammation and rapid death. This explains why people can die from viral infections while other organs remain relatively unaffected.

The brain is shielded from the rest of the body by the blood-brain barrier, which consists of tight junctions in blood vessel walls. This barrier prevents many compounds and immune cells from entering the brain, which evolutionarily protects the brain from harmful substances. However, this same barrier limits immune access to brain tumors, making immunotherapy more challenging. The concept of 'immune privilege' is now considered outdated, as the immune system does gain access to the brain through alternative pathways, though not through the same mechanisms as in other tissues.

The brain is the body's most priority organ, consuming the majority of energy, blood, and oxygen. It is described as an 'immunologically privileged' site, meaning it should not have high immune responses or active immune warfare within it. The blood-brain barrier (BBB) is the protective boundary that controls what substances can enter the brain from the bloodstream. It acts as a highly selective filter, allowing only beneficial substances while blocking harmful ones like most antibiotics and chemotherapeutic drugs. The BBB is formed by endothelial cells connected by specialized protein structures called 'tight junctions' (tarcaklar), which create a highly restrictive barrier. These tight junctions can become damaged in various conditions including epilepsy, multiple sclerosis, and neurodegenerative diseases. The gut and brain share similar barrier structures, and when gut barriers become damaged (leaky gut), harmful substances can enter the bloodstream and potentially reach the brain, causing neuroinflammation.

The blood-brain barrier (BBB) is a selective semi-permeable membrane that regulates molecule and ion movement between the blood and the brain, originally discovered by Dr. Paul Ehrlich in 1882 using trypan blue dye injections; its primary functions include maintaining a stable microenvironment for neurons by protecting them from blood chemistry fluctuations (hormones, metabolites, glucose, ionic levels), providing protection against toxins, harmful drugs, and microbes, and helping maintain appropriate neurotransmitter concentrations in different brain regions; structurally, the BBB consists of continuous capillaries with tight junctions between endothelial cells (made of proteins like claudins and occludins) that block paracellular passage, combined with minimal vesicular transport, creating a neuroprotective layer that prevents free movement of most substances between blood and central nervous system.

The blood-brain barrier is a protective layer that prevents many substances from entering the brain from the bloodstream. This barrier makes it difficult for medications to reach the brain, which is why developing drugs for brain disorders is particularly challenging. The barrier exists to protect the brain from harmful substances and maintain a stable internal environment.

The endocytic pathway involves multiple compartments: early endosomes (acidic, for sorting), late endosomes (more acidic, for further sorting), and lysosomes (most acidic, for degradation). Cargo is sorted at early endosomes based on receptor-ligand interactions. Receptors are recycled back to the plasma membrane while cargo proceeds to lysosomes for degradation. This pathway is essential for nutrient uptake, receptor regulation, and cellular waste disposal.

Phagocytosis is one of three pathways terminating in the lysosome, alongside receptor-mediated endocytosis and autophagy. Professional phagocytes (macrophages, dendritic cells) have specialized receptors, while non-professional cells (epithelial, fibroblasts) can also perform phagocytosis. The process involves: (1) adhesion via ligand-receptor interactions; (2) internalization driven by actin cytoskeleton; (3) intracellular trafficking; (4) phagosome maturation. Receptors include opsonin-dependent (immunoglobulins, integrins) and opsonin-independent (mannose receptors, lectins). Cells avoid phagocytosis through tyrosine phosphatase recruitment and signaling antagonism.

Endocytosis involves cells taking in materials through vesicle formation. Receptor-mediated endocytosis uses specific receptors that bind ligands (like LDL cholesterol) and internalize them. The acidic endosome environment causes receptor-ligand dissociation, allowing ligands to be degraded in lysosomes while receptors are recycled to the cell surface. Phagocytosis is a specialized form where cells engulf large particles like bacteria into phagosomes that fuse with lysosomes to form phagolysosomes for destruction.

Cells internalize materials through phagocytosis (engulfing large objects) and endocytosis (coated vesicle formation). Endocytic vesicles fuse with endosomes, which can take complex shapes when receptors are internalized. Endosomes fuse with lysosomes, where low pH and enzymatic activity degrade internalized materials. Phagosomes are converted to large lysosomes by fusion with lysosome-derived vesicles carrying digestive enzymes. This endocytic pathway is the opposite of the secretory pathway, converting external materials into internal cellular components.

Endocytosis brings extracellular material into cells through membrane invagination, classified into three types. Receptor-mediated endocytosis uses clathrin-coated vesicles for selective uptake of specific ligands (e.g., LDL cholesterol uptake via LDL receptors). Caveolae-mediated endocytosis forms small (~100 nm) vesicles in cholesterol-rich membrane regions, internalizing fluid and dissolved molecules non-selectively. Phagocytosis involves receptor recognition of large particles (bacteria), pseudopodia extension, and formation of phagosomes that fuse with lysosomes for digestion. Autophagy degrades damaged organelles: organelles are surrounded by ER-derived membranes forming autophagosomes that fuse with lysosomes. Chaperone-mediated autophagy selectively degrades cytosolic proteins with exposed KFERQ motifs recognized by lysosomal membrane proteins. After digestion, nutrient-rich components are recycled, while indigestible residues are expelled via exocytosis.