Systemic lupus erythematosus (SLE) – causes, symptoms, diagnosis & pathology

Systemic lupus erythematosus (SLE) – causes, symptoms, diagnosis & pathology


Alright, “systemic lupus erythematosus,”
k we totally got this. “Systemic” is easy, and refers to affecting multiple organs in
the body. “Erythematosus” means reddening of the skin, alright alright. “Lupus”
is latin for “wolf”. So affects multiple organs wolf…reddening of the skin? Not exactly,
the modern use of lupus usually refers to a variety of diseases that affect the skin…which
was possibly originally used since these diseases resemble a wolf bite on the patients’ skin.
Is that true? Who knows. At any rate, systemic lupus erythematosus, or SLE, sometimes just
lupus, is a disease that’s systemic, and affects a wide variety of organs, but notably
often causes red lesions on the skin. But how does lupus affect all these organs?
Well usually the immune system protects the body’s tissues from invaders, but lupus
is an autoimmune disease, which means that immune cells start attacking the very tissues
their supposed to protect. With lupus, essentially any tissue or organ can be targeted. And just
like a ton of other autoimmune diseases though, it’s not completely clear why it develops,
and like most diseases it’s the result of both genetics and the environment. Alright so let’s go over a specific scenario
to show how this plays out. Let’s say this guy has susceptibility genes—genes that
make him susceptible to getting lupus, and he’s exposed to UV radiation in sunlight,
which we know is an environmental risk factor for lupus. Well, given enough UV rays, think
like sunburn, the cell’s DNA can become so badly damaged, that the cell undergoes
programmed cell death, or apoptosis, and it dies. This produces all these little apoptotic
bodies, and exposes the insides of the cell, including parts of the nucleus, like DNA,
histones, and other proteins, to the rest of the body. Well those susceptibility genes
specifically have an effect on this person’s immune system such that their immune cells
are more likely to think that these are foreign, or antigens, and since they’re from the
nucleus, we call them nuclear antigens, and immune cells try to attack them. Not only
that though, susceptibility genes also cause this person to have less effective clearance,
essentially they aren’t as good at getting rid of the apoptotic bodies and so they end
up having more nuclear antigens floating around. This means that B cells might swing by, see
them, and start the production of antibodies against these pieces of nucleus, which are
called antinuclear antibodies, and these guys are present in almost all cases of lupus.
Alright so those antinuclear antibodies bind to the nuclear antigens, forming antigen-antibody
complexes. These complexes can get into the blood and then drift away and deposit or stick
to the vessel wall in all sorts of different organs and tissues like the kidneys, skin,
joints, heart. Deposited complexes then initiate a local inflammatory reaction, which causes
damage through the activation of the complement system, which, after a huge cascade of enzyme
activation, leaves cell membranes with channels that let fluid and molecules go in and out
all willy nilly, causing the cell to burst and die, usually though you’d want this
to happen to foreign cell or an infected cell, not healthy cells. When tissues become damaged
as a result of these immune complexes, it’s known as a type III hypersensitivity reaction.
UV radiation isn’t the only way to damage cells, though, right? It therefore isn’t
the only trigger that’s thought to be associated with lupus—other potential triggers that
have been associated with SLE include cigarette smoking, viruses, bacteria, use of certain
medications like procainamide, hydralazine, and isoniazid, as well as sex hormones, particularly
estrogen, which might be partly why lupus is more common in women, especially considering
it’s about 10 times more common in women than men during reproductive years, but only
about 2 or 3 times more common in childhood or past the age of 65. Okay okay, as a quick recap, the model that’s
generally thought to be what leads to SLE starts with some environmental trigger, which
damages cells, and causes apoptosis and the release of nuclear antigens. At this point,
the genetic components come in, and the person likely has certain genes that make them not
so good at clearing these apoptotic bodies and nuclear antigens, so you end up with a
lot of nuclear antigens floating around. In combination, they probably also have genes
that cause their immune cells to recognize these nuclear antigens as foreign, which initiates
an immune response, creates antinuclear antibodies that bind to antigens and then float around
and deposit in various tissues, which causes inflammation. These deposits and inflammation seem to be
the cause of most of the symptoms of lupus, which remember is a type III hypersensitivity
reaction. Many patients, though, also develop antibodies targeting other cells like red
and white blood cells, and molecules like various phospholipids, which can mark them
for phagocytosis and destruction, leading to additional symptoms. This is considered
a type II hypersensitivity reaction, although it isn’t fully understood why some of these
antibodies targeting specific cells and molecules develop. So the classic presentation of lupus is fever,
joint pain, and a rash in a woman of childbearing age, but the actual diagnosis is difficult
because it can affect a variety of people of different genders and ages, and there’re
also a wide variety of symptoms. There are general symptoms like fever and weight loss,
as well as specific symptoms depending on the specific organ system being affected and
damaged. In fact, it’s so unpredictable that a diagnosis is given only when 4 or more
out of eleven diagnostic criteria are met. The first few have to do with the skin and
often happen to sun-exposed areas; the first is a Malar rash, meaning a rash over the cheeks
that spares the nasolabial folds, sometimes just called a “butterfly rash” and this
appears after sun exposure. Second is a discoid rash, which is chronic rash in sun-exposed
areas that are plaque-like or forms a sort of patchy redness and can scar. Finally, a
general photosensitivity of the skin—essentially a catch-all category for other rashes that
happen to sun-exposed areas—typically only lasting a couple of days. Another type of tissue that can be damaged
is the mucosa, or the the inner membrane of various tissues can become damaged as well,
so the fourth criteria is ulcers in the mucus membrane of the mouth or the nose. Lupus can
also affect the serosa, which is like the outer membrane of an organ or tissue, so if
it gets inflamed, people get get serositis, which could manifest as something like pleuritis,
which is inflammation of the lining around the lungs and chest cavity, or as pericarditis,
inflammation of the lining of the heart. Although this isn’t strictly a criteria, it’s worth
noting that lupus can affect any layer of the heart—meaning in addition to inflammation
of the pericardium, they might also have inflammation of the endocardium and myocardium, leading
to endocarditis and myocarditis, of which the former presents as Libman-Sacks endocarditis,
where vegetations form, which are essentially clumps of fibrin, a blood-clotting protein
and immune cells, around the mitral valve. Next, if the joints get inflamed, patients
may also develop arthritis, specifically two or more joints to meet the criteria. If the
kidneys are affected, patients might develop renal disorders, like abnormal amounts of
urine protein or diffuse proliferative glomerulonephritis, inflammation of the glomeruli. For reasons
that aren’t completely understood, some autoantibodies that target receptors in the
brain have been implicated as well, and this can cause neurologic disorders like seizures
and psychosis. Sort of along the same lines, patients can have autoantibodies against components
of the blood, causing various hematologic disorders, for example they’ll get anemia
if red blood cells are targeted, thrombocytopenia if platelets are targeted, and leukopenia
if white blood cells or immune cells are targeted. That last one is really a mind-bender because
it means that your immune system is attacking your immune system. Alright so the last two
have to do with specific antibodies being found in the blood, the first one being antinuclear
antibody, which we already went through. Now a large proportion of patients with lupus
have these, meaning this test is very sensitive, but this test isn’t very specific, since
these are seen in other autoimmune diseases. Finally, they can have some other self-directed
antibody that isn’t antinuclear antibody, which can be one of three types. It could
be anti-Smith, which is an antibody against small ribonucleoproteins, or it could be anti-dsDNA
which is against double stranded DNA and is often seen more during periods of active disease.
These two are relatively specific for lupus. A third type of antibody though is anti-phospholipid
which is actually against proteins that are bound to the phospholipids, and is less specific
for lupus, meaning that it can pop up in other situations. There are three types of antiphospholipid
antibodies – anticardiolipin, which can cause a false-positive test for syphilis since anticardiolipin
antibodies are also sometimes involved in syphilis, the other two are lupus anticoagulant
also known as lupus antibody, and anti-beta2 glycoprotein I. Sometimes, because of these,
patients with lupus develop an antiphospholipid syndrome, where the antiphospholipid antibodies
cause a hypercoagulable state, meaning they’re more prone to developing clots and having
complications like deep vein thrombosis, hepatic vein thrombosis, and stroke. These patients
often end up needing lifelong anticoagulation therapy. So lupus is characterized by periods of flare-ups
and periods of remittance, so treatment is often aimed at preventing flares or limiting
how severe they are when they do happen. To help prevent flares, some patients may have
to avoid sunlight exposure with hats and long-sleeved clothes. To reduce severity of flares, corticosteroids
may be used to help limit the immune response, and finally, if symptoms are really severe,
certain immunosuppressive drugs might be used.

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