Clinical aspects of the complement system in systemic lupus erythematosus

General comments

The complement system is a double-edged sword in lupus ( Fig. 14.1 ). On the one hand, in its absence, a systemic autoimmune disease develops. Thus a complete deficiency of C1q, C1r, C1s, C4, or C2, all early components of the classical pathway (CP), represents a single protein deficiency state that causes lupus. Over 80% of patients with a total C1q or C4 deficiency develop SLE or a lupus-like disease. This pivotal observation, now nearly five decades old, obviously has important implications relative to the etiology of this enigmatic disease.

On the other hand, complement activation primarily as a result of the abundant immune-complexes (IC) characteistic of SLE contributes to tissue damage. The latter is reflected by the fact that reduced serum complement levels facilitate establishing a diagnosis of SLE and then become valuable biomarkers of disease activity. Complement fragment deposition, especially in the kidney, and reduced serum C4 and C3 are associated with more active disease. A return to normal levels with treatment is characteristic of a beneficial response and a much improved short- and long-term clinical outcome. Thus “too little or too much” of this immune system underlies both the etiology and pathogenesis of SLE—a paradox still to be solved.

At this stage in our understanding of SLE, 122 patients have been reported to have a complete deficiency of C1q (74), C1r (12), C1s (8) or C4 (28). Larger numbers have been noted with C2 deficiency and SLE. More common are genetic variants in C4 that result in a functional or partial insufficiency , which increase disease predisposition. Moreover, an acquired process is operating in most patients with end-organ damage mediated by activation of the classical pathway (CP) secondary to IgG autoantibodies binding to their target autoantigens.

The goal of this article is to authoritatively summarize the current state of the art and outline what are likely to become the future topics of interest relative to this paradigm of a systemic autoimmune disease process featuring complement activation. Due to the limitation in the number of references allowed, we have included more reviews of topics or an appropriate book rather than a comprehensive list of citations.

Historical notes

The complement system was discovered in the 1880s. The Nobel prize was awarded to Jules Bordet in 1919. Complement was initially thought to be a single, heat labile substance in serum that rather astoundingly could lyse bacteria in seconds. To carry this out in these early studies, complement was the lytic “factor” in blood but this reaction required a second heat stable partner, which was an acquired substance (eventually named “antibody”) to also become engaged—consequently, the term “complement”. In this rendition, the host required prior exposure to the bacteria before the lytic factor could be activated. This observation identified the CP. Over 50 years later, the alternative pathway (AP) was discovered. For almost two decades following identification of the AP in 1950s, however, it was thought to be an artefact. The AP was then “rediscovered” in the 1970s. Today, we know that the AP is the most ancient complement cascade and a key player in innate immunity. The AP is found in sponges and insects and other earlier evolutionary species including those that lack a “pumped” circulatory system. The lectin pathway (LP) was then a third major pathway identified in the 1980s. In evolution, it also preceded the CP. The LP and CP though are similar. The primary difference relates to an Ab in the CP versus a lectin in the LP triggering the system. Also, the initial set of proteases engaged are distinct but closely related as they arose by gene duplication. Following the assembly and activation of the initiation complex, the LP and CP are identical ( Fig. 14.1 , Table 14.1 ).

Currently, the complement system is undergoing a renaissance. A major contributor to this has been genetic analyses defining variants in human disease. A second factor is the FDA’s approval of a mAb to C5 to treat complement-mediated disease; namely, paroxysmal nocturnal hemoglobinuria (PNH) in 2009 and then atypical hemolytic uremic syndrome (aHUS) in 2013. In addition, the resolution provided by the crystal structures for the native proteins and the multimolecular complexes involving C3, C4, and C5 and their associated proteins, the convertases, not only helps understanding the molecular mechanisms of complement activities but also provides new impetus to drug development through complement inhibitors.

Serum levels and hemolytic activities for C4 and C3

The advantages in the clinical setting of nephelometric methodology to obtain antigenic measurements of C4 and C3 include its simplicity, rapid turnaround and accuracy. They are now time-honored and helpful tests. The CH ₅₀ or AH ₅₀ measures the lysis of red blood cells (RBCs) by the respective pathway and thus are functional tests. For example, the CH ₅₀ (aka, THC for total hemolytic complement) tests the CP as all 9 of its components (C1–C9) must be present to efficiently lyse the sensitized (Ab-coated) RBCs in the test mixture. A CH ₅₀ of 200 means that at a dilution of 1:200, a serum lysed 50% of the RBCs in the test mixture. Both CH ₅₀ and AH ₅₀ are technically demanding functional assays that are primarily of value in the screening for a complete deficiency of a component.

In the 2012 Systemic Lupus International Collaborating Clinics (SLICC) classification criteria for SLE, low complement (meaning reduced C4 and C3 antigenic levels) had a sensitivity of 59% and a specificity of 93%. The combination of positive anti-DNA Ab test and a low complement has over 90% specificity and sensitivity for the diagnosis of SLE. Likewise, an ANA titer of ≥1/640 by immunofluorescence (IF) microscopy and a low C3 also indicate that the disease process is likely SLE.

In one-third to one-half of SLE patients, the C4 and C3 will be found to be low ( Table 14.2 ). The interpretation is that IC are activating the CP, consuming these components faster than they can be synthesized by the liver (often despite increased synthesis as part of the acute phase response). An example of this remarkable degree of complement turnover is the “old” lupus band test in which IgG and C3 are detected by IF on a biopsy of uninvolved skin at the dermal-epidermal junction in over 50% of lupus patients. Likewise, deposition of C1q and fragments of C4 and C3 are commonly observed in involved organs - kidney, synovium, skin, pleura and pericardium as well as on the surface of peripheral blood cells (erythrocytes, platelets and lymphocytes). These longstanding observations reflect substantial and continuous systemic turnover of complement in this disease. Why there is not inflammation at “uninvolved” sites (for example, in the clinically appearing normal skin) is not clear. One explanation though is that complement regulatory proteins are sufficient to control the activation so that a pathologic clinical condition does not result. Thus low complement levels facilitate establishing the diagnosis of SLE. They correlate with high titer ANAs and, especially, anti-dsDNA Abs. To summarize, the pattern of low C4 and C3 represents CP activation by IC and is strongly associated with end organ damage, especially glomerulonephritis (GN). Obviously, a normal C4 and C3 does not rule out lupus as ∼50% of patients will have serum values within the normal range.

CH ₅₀ is usually only particularly informative if it is unmeasurably low (<10 hemolytic units). In this situation, it could be a sign of C1q, C4, or, more likely, C2 deficiency (1 in 100 to 200 lupus patients will have such a total deficiency of C2, Table ww ). Rarely are the C4 and C3 so consumed by IC in lupus that the CH ₅₀ is <10 but this unlikely possibility is recognizable because both are proportionally very low. If on repeat measurement the CH ₅₀ is undetectable, then antigenic assay for C2 should be obtained. If normal, antigenic levels of C1q should be assessed. Because of disease implications, family concerns, and increased frequency of serious bacterial infections in these deficient patients (commonly Streptococcus pneumoniae in C1, C4 or C2 deficient patients), a repeat test is recommended including this time a functional assessment as well of the deficient component in question.

Interpretation of C4 and C3 complement tests in SLE