Microbiology filter

March 10, 2008 2:34 PM

Is it fair to say that the cells that a particular virus uses to reproduce itself are the cells where symptoms of the disease will tend to manifest?

For example, I know that HIV uses CD4 T cells to reproduce. Therefore, the functioning of the T cells is impaired leading to immune compromise and AIDS. As I understand it, dominant strains of influenza virus use the epithelial cells of the throat and lungs to reproduce. Is that why the flu tends to cause coughs, snot, and sore throats?

I’m wanting to make sure I understand this right before I use this info for some health ed stuff I’m writing.

posted by serazin to health & fitness (14 comments total)

For those examples, you're correct.

One counter example may be that VZV (which causes chicken pox) causes symptoms in epithelial cells, but also infects nerve cell bodies. It lies dormant there, and if it reactivates later in life, it causes shingles.
posted by chrisamiller at 2:54 PM on March 10, 2008

Huh. But VZV does actually cause nerve pain when it's manifesting as shingles right? So does a virus have to directly invade a cell to cause trauma or whatnot to that cell? (I mean besides the systemic symptoms we experience like fever that are caused by complement)

I just took micro last semester but there's something obvious I feel like I'm still not getting.
posted by serazin at 3:01 PM on March 10, 2008

Well, a poliovirus infection of the central nervous system can lead to muscle paralysis, and the paralysis can in turn lead to skeletal deformities, with neither muscle or skeletal cells being infected.
posted by mr_roboto at 3:08 PM on March 10, 2008

But VZV does actually cause nerve pain when it's manifesting as shingles right?

Yes, but many people never develop shingles. I'm just pointing out that when the virus infects nerve cells and lies dormant, it's a case where the cells that the virus is infecting aren't showing immediate symptoms. You're right that in chicken pox, the cells that are churning out copies of the virus are the epithelial cells.
posted by chrisamiller at 3:11 PM on March 10, 2008

You're confusing direct damage and indirect damage. Direct damage is HIV killing out T cells.

Symptoms are observable by other people. So other symptoms of HIV include persistently englarged lymph nodes and fever. Both of those are your body's response to the infection - they're indirect damage, if you feel icky and you aren't up to speed.

If a virus lyses a cell, that's direct damage. If your natural killer cells are spraying chemicals that kill all the cells in the area, that's indirect damage. The end result is going to be an ulcer of dead cells (like a lawn on a plate). If that ulcer is big enough, it's going to take down entire organs even if the virus only attacked a few cells.

So chrisamiller says that there can be virally infected cells that don't show symptoms, and I'm saying there can be noninfected cells that do show symptoms

What you can say is that the portal of entry is usually related to the portal of exit. If you inhale a bug, you probably spread it through exhaling. If you eat a bug, you probably spread it through diarrhea.

Microbes don't penetrate bodies well, in general. When you have to get in and get out before the host system kills you, why bother penetrating the GI tract? Inhale, infect the respiratory tract, and get out.

Snot, sneeze, and cough are all our responses to irritation. It's our body's attempt to physically remove the microbe from our system. When our diaphragm hurts because we coughed too much, that was indirect damage. It's just one of the things about the way the world works that our attempt to cure ourselves puts other people at risk.

People who died from 1918 flu died because their own systems flooded their lungs, not because the virus ate so many lung cells.

In general, the symptoms are going to be centered on the site of the damage, sure, but it's easy to think of exceptions.

I'm kind of confused by your question, so I hope that helped and isn't too far off base.
posted by arabelladragon at 3:27 PM on March 10, 2008

A lot of symptoms are due to our immune reaction to the viruses. The common cold for instance affects the cells in our nasopharnyx (back of the nose/mouth) but it's the cytokines/inflammatory mediators released by our immune cells that dribble down the back of the throat that causes the inflammation and hence, sore throat. A lot of others worth the same way. HIV though, decreases the CD4+ T cells, which are essential to a proper immune response. So with HIV, it's other things that infect the person that a normal immune system can defend off that eventually kill an HIV infected person, not HIV itself.
posted by uncballzer at 3:34 PM on March 10, 2008

As I understand it, dominant strains of influenza virus use the epithelial cells of the throat and lungs to reproduce. Is that why the flu tends to cause coughs, snot, and sore throats?

uncballzer is correct. The majority of symptoms of illnesses like colds and flus, as far as I was taught, are due to the body's attempts to rev up the immune system. There are a number of cytokines involved, but one of the biggest implicated is called interferon (IFN). Case in point: ask any person being treated with interferon for HepC, and they'll tell you horrible tales of every treatment making them feel like they have the flu.

Symptoms are observable by other people. So other symptoms of HIV include persistently englarged lymph nodes and fever.

Not to nitpick, but this is incorrect. Signs are observable by other people. Symptoms are things you feel; pain is a symptom. If I touch you some place and you wince, then you have tenderness, a sign. (This is why they're called vital signs, not vital symptoms -- we can measure them and verify them "objectively.")
posted by gramcracker at 3:50 PM on March 10, 2008

I think you are mostly right, but there are a couple nit-picky details. A counter-example might by syphilis, which manifests with skin symptoms. But it is actually the endothelial cells of blood vessels that are damaged by the pathogen. Another counter-example would be renal symptoms which could be triggered by Hepatitis B, or streptococcus pyogenes - in which it is the immune complexes that damage the cells of the kidney's glomerulus.
posted by alex3005 at 4:08 PM on March 10, 2008

Is it fair to say that the cells that a particular virus uses to reproduce itself are the cells where symptoms of the disease will tend to manifest?

No, not always.

There are viruses which insert their genetic material into cell division control regions -- areas of the cell's genome that control how the cell divides -- or disrupt other control genes and activate oncogenes.

There are other viruses which insert their genetic material into the cell, and the cell translates the viral instructions into making proteins that also disrupt cell cycle. Epstein-Barr is one example.

In both cases, the end result can be a cancerous cell which then not only forms tumors, but can spread to other parts of the body through the lymphatic system.

Often, morphologically and behaviorally, cancer cells are no longer of the same type from which they originated. Some differentiation occurs as a result of the oncogenetic alterations. Cancerous esophageal tissue, for example, behaves and looks like small intestinal tissue.
posted by Blazecock Pileon at 4:33 PM on March 10, 2008

Gram, I knew if I posted tired I'd make some horrid mistake. Thank you for the correction.
posted by arabelladragon at 4:41 PM on March 10, 2008

A counter-example might by syphilis, .. streptococcus pyogenes

T. pallidum and S. pyogenes are bacteria, not viruses.

In answer to the OP, it really isn't that simple. You have to really understand the pathophysiology of the disease to understand how it manifests. Just because a virus is only able to enter a particular type of cell doesn't necessarily mean that the manifestations of that infection will remain confined to that cell line/tissue/organ. (As others have pointed out, e.g., muscle atrophy due to deinervation.)
posted by sero_venientibus_ossa at 5:06 PM on March 10, 2008

No, because the immune reaction frequently wipes out nearby cells of a totally different type. Structural damage can have a similar effect, for example you die of multiorgan failure when ebola screws up the blood vessels. When you have viral infection of your thyroid gland, the main effect that you notice is the effects of thyroid hormone on your body. The flu and feeling like death because of the cytokines is another example of immune collateral damage. A more strange example: diphtheria is a normal bacteria which lives in your throat. When a particular virus (which is transmitted person to person) infects the diphtheria in your throat, it transforms them to make diphtheria toxin (which kills you) and otherwise behave badly. There are other viral encoded toxins, but not many that come to mind.
posted by a robot made out of meat at 6:15 PM on March 10, 2008

Oh, I should also say that the negative which you may be looking for is also untrue. There can be many infected areas where you will not notice the loss of a few (or many) cells and therefore suffer no symptoms. For example, HIV infects a variety of cell types besides helper T-cells (macrophages, dendritic cells IIRC). You just don't get as big a problem with losing a bunch of those. HIV also infects tubule cells in the kidney, and most people with HIV never develop a problem with that.
posted by a robot made out of meat at 6:20 PM on March 10, 2008

Thanks all so much. The detail in these answers in super interesting for me. I was trying to look at it from a more simplistic perspective for something I'm writing about how HIV works, but I was probably trying to get too simplistic.
posted by serazin at 10:32 PM on March 10, 2008