How Pathogens Penetrate the Blood-Brain Barrier

April 17, 2020

The blood-brain barrier (BBB) is a crucial immunological feature of the human central nervous system (CNS). Composed of many cell types, the BBB is both a structural and functional roadblock to microorganisms, such as bacteria, fungi, viruses or parasites, that may be circulating in the bloodstream. As a result, the BBB is a key regulator of microorganism entry into the CNS and exists at the interface of blood vessels and interstitial fluid throughout the brain. The BBB also exists at other intersections of the CNS and periphery, including between blood and cerebrospinal fluid-producing cells. Its purpose is to protect and regulate the brain’s microenvironment. 

Composition of the Blood-Brain Barrier

The BBB is composed of multiple cell types. These cells line the microvessels of the brain and work in concert to protect the CNS, and its neurons, from any pathogens located in the periphery.

Diagram of a microvessel in the brain lined by the blood-brain barrier. The blood-brain barrier is a multicellular, compound structure composed of endothelial cells, pericytes and astrocytes in direct contact with brain tissue.
Diagram of a microvessel in the brain lined by the blood-brain barrier. The blood-brain barrier is a multicellular, compound structure composed of endothelial cells, pericytes and astrocytes in direct contact with brain tissue.
The BBB is a compound structure following the brain’s labyrinth of vasculature. It’s composed of 4 cell types:
  • Endothelial Cells. These cells line the inside of blood vessels. At the BBB, they are closely associated with one another via tight junctions to form a barrier. These cellular junctions are crucial to the microvessels in our brains because they maintain the integrity and permeability of the vessel, thereby regulating passage through the BBB. 
  • Pericytes. Embedded into the basement membrane of microvessels, pericytes associate closely with endothelial cells at the BBB. Pericytes are thought to be derived from a common precursor to smooth muscle cells, and while they lend structural support to microvessels, they also signal with endothelial cells to influence permeability and growth.  In the brain, pericytes may also perform immune cell-like functions such as sensing, engulfing and destroying potentially harmful blood-derived microorganisms.
  • Astrocytes. Astrocytes, named for their star-like shape, are support cells that contribute to structural properties of the BBB. Astrocytes are known to recruit peripheral cells, such as white blood cells, into the CNS through the BBB. 
  • Microglia. As the resident immune cell of the CNS, microglia sit just beyond the BBB. Although they are not typically considered part of the BBB, microglia survey the CNS for microbes and have the capabilities to engulf and destroy those they encounter. Therefore, microglia are another line of immunological defense against potential pathogens or toxins crossing the BBB.

Getting Through the Blood-Brain Barrier

The BBB is effective in protecting the CNS, but as with many barriers, it is not perfect. There remains a healthy debate as to whether the CNS is truly an immunologically-privileged site because it’s not impenetrable to peripheral cells and microorganisms. This permeability raises questions. How do microorganisms enter the CNS? What microorganisms can cross the BBB?

Three mechanisms for microorganism transfer across the blood-brain barrier: A) transcellular route, B) paracellular route and C) infected phagocyte route (Trojan Horse).
Three mechanisms for microorganism transfer across the blood-brain barrier: A) transcellular route, B) paracellular route and C) infected phagocyte route (Trojan Horse).
Transcellular CNS Penetration

Microbes that cross the BBB through the transcellular method cross into the CNS through endothelial cells. They gain access to the luminal side of the blood vessel endothelium, where they traverse through the endothelial cells themselves. Once they’ve crossed the barrier, these microbes exit through the other side of the cell that’s in direct contact with astrocytes, microglia and neurons. 

There are 2 mechanisms of transcellular CNS penetration: absorptive-mediated and receptor-ligand mediated. Absorptive-mediated transcytosis (AMT) relies upon charge interactions instead of specific ligand-receptor binding. In AMT, non-specific interactions with the endothelial membrane result in the absorption of a protein, molecule or microbe directly into the endothelial cell. It is then transported across the cell and released into the CNS. 

In contrast, receptor-ligand mediated transcytosis (RMT) requires specific binding between the microbe (ligand) and endothelial cell (receptor). Although the absorptive processes for AMT and RMT are similar, host-pathogen interactions require a much higher specificity, thereby limiting the capability of microbes to enter endothelial cells through RMT. Receptors that facilitate RMT include the transferrin receptor, insulin receptor and low density lipoprotein receptor-related proteins 1 and 2 (LRP1 and 2). 

Escherichia coli is a popular model for studying microbial transfer across the BBB. Most strains of Ecoli are not dangerous, however, particular strains of E. coli, such as Ecoli K1, have the unique ability to evade a host’s immune response and reach a high level of bacteremia, which can result in the development of bacterial meningitis. E. coli enters brain microvascular endothelial cells (BMEC) primarily through RMT using a handful of receptors, resulting in host-pathogen binding between the E. coli and BMEC. This pathology is seen most commonly in newborns and can be transferred from mother to child during birth. 

Paracellular CNS Penetration

Microbes that cross the BBB through the paracellular method pass between endothelial cells, as the prefix 'para,' meaning ‘alongside,’ suggests. In both transcellular and paracellular CNS penetration, microbes must attach to BMEC before they are transferred. In this scenario, a microorganism attaches to a BMEC and enters the CNS between two endothelial cells. Tight junctions, the anchors that hold adjacent endothelial cells close together, are disrupted during this mechanism of microbial transfer. 

Compared to transcytosis, fewer microorganisms use paracellular transport to enter the CNS. Treponema pallidum, the bacterium responsible for syphilis, invades the nervous system during early infection. The bacterium is present in intercellular junctions of aortic endothelial cells, suggesting T. pallidum invades tissues paracellularly. While the microbe ligand and endothelial cell receptor required for initial binding of T. pallidum are unknown, T. pallidum seems to interact with platelets to influence endothelial cell permeability and facilitate BBB transfer.   

Infected Phagocytes (Trojan-Horse Method)

In contrast to the direct movement of a microorganism across the BBB in trans- and paracellular microbial transfer, the Trojan-Horse method is an indirect form of microbial transfer. The BBB is permeable to phagocytic white blood cells, which regularly circulate in the blood to provide immunological surveillance, migrating in and out of tissues. Some microorganisms co-opt this natural process and use it to their advantage. In the Trojan-Horse method, microbial transfer occurs with the transmigration of an infected phagocyte. As an infected white blood cell crosses the BBB, the microorganism also gains access to the CNS.

Human immunodeficiency virus-1, HIV-1, is a lentivirus that enters the CNS shortly after systemic infection. Although multiple hypotheses exist on how HIV-1 enters the CNS, the frontrunner is that the virus accesses the CNS through the Trojan-Horse mechanism. The virus is well-known to infect host white blood cells using the CXCR4 and CCR5 receptors. Infiltrating, infected monocytes may be the primary carrier of HIV-1 through the BBB.  

It’s important to note that these methods of microbial transfer are not mutually exclusive, and microorganisms can use more than 1 route to enter the CNS.

Therapeutics and the Blood-Brain Barrier

Meningitis, syphilis and AIDS are 3 important causes of death worldwide. All 3 are caused by microorganisms that are capable of entering and infecting the CNS, but unfortunately, the BBB’s propensity for protection also acts as a hurdle to treatment. 

Therapeutic approaches developed in the last decade exploit existing properties and mechanisms of the BBB. For example, researchers are attempting to deliver neuropharmaceuticals into the CNS using delivery vectors that target receptors on BMECs that are involved in RMT of microbes. Neuropharmaceuticals can also be packaged into biodegradable nanoparticles. This technique also uses existing RMT pathways to gain entry to the CNS and can be further targeted for tissue-specific uptake. 

Neurotherapeutic design involves an understanding of neuroimmunology at the BBB and throughout the brain. A better understanding of the BBB and microbe entry mechanisms, along with new insights into the brain’s complex immune system, will ultimately aid in the development of effective neurotherapeutics.

Author: Taylor Evans

Taylor Evans
Taylor is a Ph.D. candidate in the Cellular and Molecular Medicine Training Program at Johns Hopkins School of Medicine.