Complement System

There are 3 principle pathways involved in complement:

1. Classical pathway 2. Alternative pathway 3. Mannose-binding lectins pathway

SO: Classical pathway begins with activation of ______ when it recognizes the CH2 region (constant region heavy chain) of either ______ or ______ bound to pathogen surface. Alternative pathway is activated by ___________ spontaneously. And finally, the mannose-binding lectin pathway is activated when either ______ or ______ recognize _____ motif on pathogen surface

1. Classical: activation of C1 when it recognizes CH2 region of either IgM or IgG bound to pathogen surface. 2. Alternative: activated by hydrolysis of C3 (C3 —> C3-H2O) spontaneously (no stimulus needed for this step). If it does not find foreign surface, gets inactivated 3. MBL pathway: activated when either MBL or Ficolins (similar to MBL) recognize SUGAR motif on pathogen surface

We know that C3 convertase leads to production of various fragments that contribute to 3 major functions of complement: peptide mediators, binding complement receptors on phagocytes, and forming membrane-attack complex. Which fragments are important for each?

1. Peptide mediators to induce inflammation: C3A, C5A, and C4A (to a lesser extent) 2. Binding complement receptors on phagocytes to activate them: C3B 3. membrane-attack complex (lysis of pathogens via osmosis): C5B, C6, C7, C8, and multiple C9's

Thus far we see complement plays a role in phagocytosis (via opsonization), inflammation (via "A" fragments releasing chemotactic factors and activating other immune cells), and direct killing of pathogen (via membrane attack complex). The complement system has 3 other major functions: Activation of ________ cells and maturation, removal of _________ from circulation, and modulation of _______

Activation and maturation of B cells, removal of immune complexes from circulation (to prevent continued inflammation after infection has passed), and modulation (i.e. shutting down) of T cells (after infection has passed)

Thus, unlike alternative and MBL, the classical pathway is an _______ pathway

Adaptive (since it requires IgM or IgG to be bound to pathogen surface)

What a re the 2 pathways that are innate (i.e. do not require presence of antibody)?

Alternative and MBL (recall that classical DOES require antibody like IgG or IgM, so it is not innate)

Which pathway generates the most C3B?

Alternative pathway

Useful diagram of the understanding complement and the 3 pathways

As we see there are 3 pathways that each get activated by different things. But all 3 involve formation of a C3 convertase (thought he C3 convetase for alternative looks different from that of classical and MBL). This also lists the 6 major functions of complement cascade (recruitment of inflammatory cells, opsonization of pathogens, killing of pathogens via MAC (membrane attack complex), B cell triggering and maturation, T cell modulation, and immune complex removal

Why do you only need 1 IgM but TWO IgG's for C1Q to bind?

Because C1Q has multiple heads that must bind to antibody. Since IgM is a pentamer, there are plenty of sites for C1A to bind. But since IgG is monomeric, NEED two of them in close proximity for C1Q's heads to bind

Why does IgA not activate complement?

Because it functions to protect mucosal surfaces, and like we said before we do not want complement activation at mucosal surfaces because that would damage the surface and defeat the whole purpose of the surface being there!

Why does IgE not activate complement?

Because majority of IgE antibodies are found bound to mast cells. So the Fc region of the IgE is not available for binding of complement

Why is it crucial that complement is NOT activated on barrier epithelia (skin and mucosal surfaces)?

Because those are the barriers that we want to remain INTACT, and if we activate cell destruction it would do more harm than good

HOWEVER, you can also have SYNERGISTIC activity, where both ______ and ______ bind the bacterium and the macrophage

Both C3B and IgG bind. You can think of this as "double opsonization", more effective cuz you're combining two different methods to ensure phagocytosis. Note that you do not need C5A here.

We also know that complement functions to induce inflammation. Which fragments are crucial for this?

C3A and C5a (and to a lesser extent, C4A). These are small molecules (recall that "A" fragments are small chemical mediators) that release chemotactic factors and other molecules that bring the right immune cells to the right place

So, the 3 chemical mediators in complement that induce inflammation are:

C3A, C4A, and C5A

So, _________ binds to antigen-antibody complexes, which is then recognized by _____ receptors on red blood cells. The RBC's then take the complex to the ______ and ______ where they are engulfed by macrophages

C3B binds antigen-antibody complex, and the C3B is recognized by CR1 receptors on RBC's. RBC's take Complex to liver and spleen, where complexes are engulfed by macrophages

So in order to get phagocytosis of a bacterium using alternative or MBL pathway, it requires two steps: _______ and _____

C3B binds, AND C5A binds

How are immune complexes REMOVED from circulation via the complement system?

C3B is responsible for this. Many C3B's will bind to the antibody-antigen complex, which then binds to CR1 receptors on red blood cell surfaces. The RBC's then take those C3B-bound antigen-antibody complexes to the liver and spleen where macrophages will engulf and remove the complexes, and let the RBC's go free.

What are the two important opsonins in complement (i.e. fragments that bind to pathogen to mark for phagocytosis)?

C3B is the major one, and C4B also plays a role

While the classical pathway is activated via recognition of Fc region of antibodies (specifically, _______ and _______), the mannose-binding lectin pathway is activated by recognition of _______ on pathogen surface

Classical: IgG and IgM. Mannose-binding lectin pathway: recognition of SUGAR motifs (carbohydrate motifs) on pathogen surface

Components of the complement are cleaved (at many steps in the cascade) into two pieces: _______ and ______

Cleaved into "b" fragment (for binding, big) and "a" fragment (soluble mediator, small)

The first antimicrobial system activated when microorganisms breach the barrier epithelium is the _______

Complement system

However, if you have ANTIBODY present, all you need is one thing: __________-linking of _______ IgG molecules on macrophage

Cross-linking of two IgG's. Once you have this, it is sufficient to induce phagocytosis. No need for any complement proteins in this method.

Fragments that function as peptide mediators can cause mast cells to ________, thus releasing their contents and inducing inflammation

DEGRANULATE

Which classes of antibody DO activate complement, and which do NOT?

DO: IgM, IgG1, IgG2, and IgG3 DON'T: IgA1, IgA2, and IgE

Hereditary angioedema

Even the slightest trauma initiates the complement pathway, and there is no inhibition of C1 to prevent the pathway from progressing more and more

Where does complement function and where does it NOT function?

Functions on the internal surfaces of the body. Does NOT function on barrier epithelia (like mucosal surfaces and skin surfaces)

What is another major opsonin (think in terms of antibodies)?

IgG

However, while the majority of complement components are synthesized in the liver, others are synthesized by the ________ themselves

Immune cells themselves! Includes mast cells that make C1Q, monocytes and macrophages making a whole bunch of components, dendritic cells masking a whole bunch, etc. Not sure how much of this we need to memorize...

One of the major pathological features of lupus is the deposition of _______

Immune complexes

Since we also have IgM and IgG circulating in our blood, what is preventing complement activation (specifically the classical pathway) from occurring within the circulation itself?

It has to do with the conformation of the IgM and IgG. The conformation that they have when freely circulating does not expose the binding site for C1Q. Only when the antibody is BOUND to pathogen surface does true binding site for C1Q become available. Specifically, the antibody takes on a "staple" form that exposes the binding site for C1Q

Most of the components of the complement cascade are synthesized in the _____

Liver

In cases of autoimmunity, we can generate _______ complexes on our own cells

Membrane attack complexes (MAC's)

However, many bacteria have evolved ways to mimic the _______ and shut down complement in the host

Mimic the regulators

Of the skin and mucosa, which is a more common surface route for infectious agents?

Mucosa, since it is often only one cell thick while the skin is much thicker and has layers of dead cells

What is the membrane-attack complex?

Multiple fragments (C5B through C9) come together to form a pore and this enables pathogens to be LYSED by osmotic lysis (water and ions flow in, lose the pathogen)

Patients that have deficiencies in the terminal complement components (i.e. membraneattack components) tend to have much higher risk of developing infections from the ______ bacteria

Neisseria

Let's say we have a bacterium that has C3B bound to it via alternative and MBL pathways. Is binding of that C3B to a macrophage receptor enough to trigger full phagocytosis of the bacterium?

No. For alternative and MBL, need TWO things to happen. First, C3B must bind to bacterium surface and to receptor on macrophage (which has already happened in the above case). However, you ALSO need C5A to bind to receptor on macrophage as well.

Let's say the complement cascade does not form the MAC. Was the cascade useless?

No. This is NOT the only function of the cascade! You have still generated opsonins, you have still generated chemoattractants tha bring immune cells to the site. So def not useless even without MAC.

Components of the complement system are not only in the ________ system, but also found in the tissue________

Not only in circulatory system, but also in tissue fluid (so in the dermis and lamina propria, we have the necessary components of the complement cascade to be activated if an organism breaches)

What activates the ALTERNATIVE pathway?

Nothing, it is spontaneously activated at low levels essentially all the time. Recall it is activated by hydrolysis of C3 to C3-H20 (pronounced "C3 water", and if it doesn't bind to a pathogen quickly, it is inactivated quickly.

In order to activate the classical pathway, you need either ONE molecule of _______ or TWO adjacent molecules of ______ bound to pathogen surface

One IgM or TWO adjacent IgG

Of the antibodies that DO activate complement, which can be transferred across placenta to neonate?

Only IgG1 and IgG3

Another really useful diagram outlining the 3 pathways in. A bit more detail

Only caveat that he mentioned (see bottom left red box). He said that a while ago, people thought that classical pathway can ONLY be activated by antibody. Now we know there are other things that can activate it like C-reactive protein and by some bacterial surfaces directly

C-reactive protein binds to _______ on the surfaces of certain bacteria

Phosphocholine

The complement system is a system of _____ proteins, some of which are inactive ______ proteases (zymogens) that are activated by ______.

Plasma proteins, some of which are inactive SERINE proteases that are activated by cleavage

Deficiencies in complement also often lead to recurrent _________ infections,specifically with those that are __________-ated

Recurrent bacterial infections, specifically those that are encapsulated

Virtually every step of complement cascade is __________.

Regulated

So, hereditary angioedema is due to a defect in the REGULATION of _____

Regulation of complement (specifically via defect in C1 inhibitor, which normally keeps complement in check)

The two most common barriers/surfaces breached by infectious agents to enter the body are:

Skin or mucosa.

How is it that complement is not activated on skin and mucosal surfaces?

They contain fluid that itself contains inhibitory molecules that prevent activation of complement

WHat is a common cause for deposition of immune complexes?

Think back to what we talked about, with C3B being responsible for removal of antigen-antibody complexes. So if you have a deficiency early in the complement cascade that prevents you from making C3B, then you can't remove the immune complexes via liver and spleen, and get deposition (leading to lupus-like disease, AKA immune complex disease)

What can antibodies do and what can't they do?

What they do: bind to stuff, clump stuff, neutralize particles, block them from blinding to receptors, etc What they DON'T DO (the main point of this card): they do NOT have intrinsic killing ability. They rely on complement cascade to allow them to do that