A host cell is a living cell that can be inhabited by a foreign entity, such as a virus, providing the environment and resources for it to multiply. These cellular interactions are a fundamental aspect of biology and disease. This article explores the nature of host cells, their relationship with pathogens, and their applications in science and medicine.
What Exactly Is a Host Cell?
A host cell provides a home and resources for an outside entity, from an infectious agent to foreign genetic material. These cells have their own complex functions before any interaction. Their outer plasma membrane acts as a gatekeeper, controlling what enters and exits. This surface, studded with molecules for communication, is the primary point of contact for invading organisms.
The diversity of host cells spans all domains of life. In humans, eukaryotic cells like the epithelial cells lining our airways or immune cells in our blood can serve as hosts. These cells contain a nucleus for their DNA and other compartments called organelles that perform specific jobs. For instance, mitochondria generate energy, while the Golgi apparatus packages proteins.
Prokaryotic cells, like bacteria, lack a nucleus and complex organelles but also serve as hosts. For example, a bacterium such as Escherichia coli can be infected by a virus called a bacteriophage. In this case, the single-celled bacterium itself becomes the host, showing this dynamic occurs at all levels of biology.
All cells are constantly active, producing energy, synthesizing proteins, and repairing damage to sustain themselves. The ability of an outside agent to co-opt these functions is what defines a cell as a host. This takeover is central to how many pathogens operate.
The Unwilling Landlord: Why Pathogens Need Host Cells
Many pathogens cannot replicate on their own and depend on the internal environment of another cell to complete their life cycle. Viruses are the most prominent examples and are classified as obligate intracellular parasites because they lack the machinery for self-replication. Outside of a host, a virus is inert and must commandeer a living cell to produce energy and build new components.
Once inside, a pathogen gains access to the host’s resources. The cell provides raw materials for building new infectious particles, including amino acids for proteins and nucleotides for DNA or RNA. The host’s metabolic pathways also supply the energy, as ATP, required for these processes. The pathogen outsources its manufacturing and energy needs to the host.
This dependency extends to the cell’s machinery. Viruses, for instance, lack their own ribosomes, which translate genetic code into proteins, so they hijack the host’s ribosomes to produce viral proteins. They also use the host’s enzymes to replicate their genetic material. This reliance is not limited to viruses; some bacteria, like Chlamydia, and protozoan parasites, like Plasmodium, are also obligate intracellular parasites.
Breaking and Entering: How Pathogens Hijack Host Cells
A pathogen takes over a host cell by first breaching its defenses. Many viruses use a specific interaction between proteins on their surface and receptor molecules on the host’s membrane. For example, the SARS-CoV-2 virus uses its spike protein to bind to the ACE2 receptor on human cells. This binding can trigger endocytosis, where the cell envelops the virus, or lead to direct fusion of the membranes, releasing the viral contents inside.
Once entry is achieved, the hijacking of the cell’s internal machinery begins. The pathogen’s goal is to replicate its genetic material and produce the proteins needed for new particles. Viral genes are transcribed into messenger RNA (mRNA) using the host’s enzymes. This viral mRNA is then processed by the host’s ribosomes, which translate it into viral proteins.
With all components manufactured, new viral particles are assembled in the cytoplasm. Viral proteins and copies of the viral genome come together to form complete virions. The final step is to exit the cell and spread the infection, which can be destructive to the host.
The final step is exiting the cell to spread the infection. One exit strategy is cell lysis, where new viruses burst through the membrane, destroying the host. Another method is budding, where a new virus pushes against the cell’s membrane, wrapping itself in a piece of it as it exits. This process creates a viral envelope and allows the virus to leave without immediately killing the cell.
Host Cells as Tools: Innovations in Science and Medicine
Beyond their role in disease, host cells are tools in scientific research and medical innovation. Scientists cultivate these cells outside an organism in a laboratory setting, a technique known as cell culture. This allows for the study of cellular processes in a controlled environment. Mammalian cell lines, like Chinese Hamster Ovary (CHO) cells, are widely used because they can be grown in large quantities and genetically engineered.
This capability is foundational to modern biotechnology. CHO cells, for example, are frequently used as living factories to produce complex therapeutic proteins. By inserting a human gene into the CHO cells’ DNA, scientists can coax them into manufacturing proteins like monoclonal antibodies, which are used to treat cancers and autoimmune disorders. Similarly, bacteria like E. coli are used as hosts to produce simpler proteins like insulin, providing a reliable and scalable source for this life-saving medication.
For many decades, influenza vaccines have been produced by growing the virus in fertilized chicken eggs. The viruses replicate within the egg’s cells, and are then harvested, purified, and inactivated or weakened to create the vaccine. More modern methods use large-scale cultures of mammalian cells, which can offer faster and more flexible manufacturing, especially during a pandemic.
In research, host cell cultures serve as model systems for a vast range of investigations. They are used to explore the fundamental mechanisms of how cells function, grow, and communicate. By infecting cultured cells with pathogens, researchers can observe the intricate details of infection and test the effectiveness of new antiviral or antibacterial drugs. These cellular models are also used in toxicology to screen new chemicals and medicines for potential harmful effects, reducing the need for animal testing.