Circulating Immune Complexes in Disease

Immune Complexes

Circulating immune complexes (ICs) have been implicated in the pathogenesis of a wide variety of diseases:

Deposition in tissues Þ inflammatory response

Interference with immune responses

How then are ICs quantified?

(1) [125 I] C1q-PEG precipitation assay – C1q* (asterisk indicates radiolabel) is added to sample containing ICs Þ incubation Þ add 2.5% PEG solution Þ C1q*-IC complexes precipitate and are quantitated.

(2) Solid phase C1q assay – Latex beads coated with purified C1q are added to sample containing ICs Þ incubation Þ beads sedimented by centrifugation Þ add anti-Ig* antibody (binds ICs) Þ ICs quantitated.

(3) Raji cell assay – same as above, but instead of using latex beads use Raji Cells (express C3 receptors (CR2)).

(4) Conglutinin assay – The cow protein conglutinin, which reacts with human iC3b, is used to coat plates. Samples containing ICs are added to plates Þ incubated Þ excess stuff aspirated Þ ICs remaining bound to plates quantitated with anti-Ig*.

Problems:

Relationship of free and cell-associated (e.g. RBC bound) complexes.
Coincidence of complexes and auto-antibodies involving the same antibody.
Relationship of positive results in different types of assays (i.e. how do the results from different assays compare?).

Examples of Immunopathology in Immune Complex disorders:

Systemic Lupus Erythematosus (SLE)

Abs are produced against a wide range of self antigens.

DNA:anti-DNA ICs deposit in the glomerular basement membrane Þ renal failure.

IC deposition causes local inflammation.

Disease monitored by: anti-DNA antibody levels, CH50 levels, and IC levels.

Polyarteritis Nodosum

A proportion of cases follow Hepatitis B infection

ICs contain viral antigens Þ deposition Þ vasculitis and nephritis.

Recovery associated with normalization of complement, loss of ICs, and appearance of free anti-viral antibody.

Subacute bacterial endocarditis

Patients exhibit manifestations of connective tissues disease (splinter hemorrhages, thrombocytopenia).

Circulating ICs contain antibodies and bacterial antigens.

Antibiotic treatment Þ IC levels decline Þ clinical improvement.

Neoplasms

ICs consist of antibody and tumor antigens.
Levels of ICs proportional to tumor mass!
ICs inhibit some cell mediated immune reactions (e.g. ADCC).

IC clearance mechanism

C3b bearing ICs bind to C3b receptors (CR1) on RBCs. After binding the RBCs the ICs are converted to inactive metabolites (iC3b and C3dg), and are then removed in the liver or the spleen. This is a homeostatic mechanism by which IC levels are kept in check.

In a disease state where IC production overwhelms this clearing mechanism Þ IC deposition Þ disease.

The complement system is coded by some segments of the HLA gene. In cases of mutation/deletion, where complement does not form properly, the CR1 receptor on RBCs won’t be able to clear excess IC production. This can lead to IC deposition Þ disease. (example: C4a deletion leads to SLE)