The Woman with SLE and Life-Threatening Pneumococcal Sepsis

Recently, I was called upon to give my opinion on an Afro Caribbean lady with well controlled SLE who had been admitted to ICU with Pneumococcal septicaemia. She was in her late 30s, had well controlled SLE with normal complements, and was on hydroxychloroquine but not steroids or other immunosuppressives. A blood film showed no evidence of Howell Jolly bodies. She had not received Pneumococcal vaccination.

Fortunately, she recovered, but it was touch and go for several weeks.

So why did the lady who was not hyposplenic, hypocomplementemic, and not on immunosuppressive agents, develop life threatening invasive Pneumococcal infection?

Invasive Pneumococcal infection is uncommon, but not rare in SLE. Around 4.7-7.1% of patients develop this infection at some point. While the other factors mentioned above may contribute, the principal risk factor is polymorphism for the FcgammaRIIa gene.(henceforth, written as FcyRIIa).

There are 3 classes of Fcy receptors- FcyRI (CD64), RFcyRII (CD32), and FCyRIII (CD16). FcvRI is a high affinity receptor that is capable of binding to monomeric Ig. The other classes are low-affinity receptors, which can only bind to multimeric or clustered Ig.

FcyRII is subdivided into FcyRIIa, FcyRIIb and FcyRIIc receptors. The first and third are activating receptors, which act through intracellular ITAM motifs, while FcyRIIb is an inhibitory receptor, that acts through ITIM motif.

FcyRI is mostly an activating receptor, although very occasionally it inhibits.

FcyRIII receptors have 2 subtypes- FcyRIIIa and FcyRIIIb. The first is present on cells of mononuclear-macrophage lineage and NK cells, while the second appears in neutrophils.

Thus, neutrophils have two principal FcyR receptors- FcyRIIa, and FcyRIIIb.

FcyRIIa has 2 alleles, depending on the presence of histidine or arginine at position 131 of the extracellular domain receptor peptide- designated as H131 or R131. They are expressed co-dominantly.

The two alleles have equal prevalence amongst Caucasians and Afro-Caribbeans. The prevalence of R131 is only 10% amongst Asians.

While all FcyR receptors bind IgG1 and IgG3 containing immune complexes well, only subjects who are homozygous for H131 at FcyRIIa bind IgG2 containing immune complexes well. Homozygotes for R131 bind IgG2 poorly, while H/R 131 heterozygotes have intermediate binding to IgG2.

This is important because IgG2 is the only class of immunoglobulins that provides protection against polysaccharide antigens present in the capsule of organisms such as Pneumococcus, Haemophilus influenzae and Meningococcus.

While IgG1 and IgG3 bind complement well, IgG2 does so poorly. Thus immune complexes containing IgG2 are inordinately dependent on Fc receptor mediated clearance, rather than complement-mediated clearance.

Several studies have shown that subjects who are homozygous for the R131 polymorphism for FcyRIIa are more susceptible to invasive Pneumococcal and other encapsulated organisms, irrespective of their complement status. Subjects who also have low complement are at even greater risk.

This is particularly true for Lupus, as the R131 allele is over-represented in Lupus- around 65%.

IgG2 antibodies are also the main subclass found in anti-C1q antibodies, thus conferring a higher risk of Lupus nephritis in R131 homozygotes. However, antibodies to C1q are uncommon, therefore the predisposition to Pneumococcal invasive infection can exist in subjects who do not have Lupus nephritis, as with the lady described above.

Similar polymorphisms exist for FcyRIIIa and FCyRIIIb.

With FcyRIIIa, present in mononuclear and NK cells, the polymorphism is F176 or V176. The former are likely to suffer more infections as they phagocytose IgG1 and IgG3 containing immune complexes poorly. Anti dsDNA and anti-nucleosome antibodies are of IgG1 or IgG3 subclass, and immune complexes containing these subclasses are cleared poorly by macrophages and monocytes, thus increasing the risk of Lupus in these subjects.

With FcvRIIIb, a GPI linked receptor present only in neutrophils, there are two alleles- NA1 and NA2. NA2 homozygotes have poor neutrophil induced phagocytosis of immune complexes compared with NA1 homozygotes and thus are at greater risk of Lupus and certain infections. Those who are homozygous for both R131 and NA2 are at risk of overwhelming meningococcal sepsis.

It is important to immunise all Lupus patients against Pneumococcus.

SLE & Complement Deficiency- Where Do We Stand?

The effect of complement deficiency is hierarchical in SLE, with 90% of those with deficiency of C1q, 80% with C4 deficiency and 30% with C2 deficiency developing the disease. While those with C1q deficiency develop classical SLE, with a high incidence of lupus nephritis, deficiency of C4 or C2 lead to unusual clinical and immunological phenotypes. Patients with deficiency of either of these two early complement components present with rash, but no nephritis, have low titre ANA, often in a speckled pattern, do not have antibodies to dsDNA, and have a high prevalence- more than 50%- of anti-Ro antibodies.

C4 and C2 are both in linkage disequilibrium with the MHC antigens. They are positioned on the short arm of chromosome 6 between HLA Class B and DR antigens and are part of HLA Class III molecules along with factor B. Homozygosity for the null allele of C2 is present in 1 in 20,000 Caucasions, and as stated above, around 30% of these subjects develop SLE.

The inheritance of C4 is unusual. Most people have not one, but two alleles of C4 on 6p, designated as C4A and C4B. The two C4 alleles are part of a 4-gene cassette called RCCX which is subject to duplication or deletion, leading to Copy Number Variation, such that individuals may have 1-8 C4 alleles across the two chromosomes. Most Caucasians have 2-4 functioning alleles. Thus, total deficiency of C4 is exceedingly uncommon.

Deletion of C4B has not been associated with lupus, but when both copies of C4A are deleted, there is a higher prevalence of lupus compared with those whose C4A copies are intact. Deletion of C4A is seen as part of extended haplotype HLA B8, SC01, DR3. (S stands for S allele of Factor B, C for C-allele of C2, 0 for null-allele of C4A and 1 is an allele of C4B). Thus, subjects who are homozygous for this extended haplotype are at greater risk of SLE.

The extended haplotype HLA B8, SC01, DR3 is one of the commonest extended haplotypes found in Caucasians, with a haplotype frequency of around 9%. Thus the likelihood of heterozygosity in the Caucasian population using the Hardy Weinberg equilibrium is just under 17%, and the likelihood of homozygosity is 0.8%.

In practice, C3 and C4 levels are measured by most Rheumatologists. C4 levels are normally much lower than C3. A low C4 may be inherited or due to active SLE with complement consumption. An isolated low C4 may thus be difficult to interpret. Total haemolytic complement, or CH50, which is an index of all complement components in the classical pathway and is often measured, will therefore also be low, and will not help in such cases. (CH50 is the reciprocal of the dilution of the patient's plasma which, when added to sheep erythrocytes coated with rabbit anti-sheep antibodies, leads to 50% lysis of the sheep RBC. CH100, which is measured in other labs, is self explanatory). However, complement activation products such as C4d will be raised in complement consumption and low in inherited deficiency without SLE. Complement activation products in plasma are heat labile and technically difficult to measure. Cell bound complement activation products hold a lot of promise.

The HLA B8,DR3 extended haplotype is also associated with Type 1 Diabetes, Coeliac Disease and Adrenal insufficiency as part of polyglandular autoimmune syndrome type II.

Interestingly, just as C4A deficiency is part of an extended haplotype, so is deficiency of the product encoded by another member of HLA Class III, 21-beta hydroxylase. CYP21B is often deleted alongwith C4B as part of two other extended haplotypes. Homozygosity for these haplotypes result in Congenital Adrenal Hyperplasia.

C2 deficiency also occurs as part of an extended haplotype- HLA B18, DR2. Around 1% of Caucasians are heterozygotes. C2 is the only complement component in the classical pathway where heterozygosity is associated with reduced C50. With the others, C50 is only low with a homozygous state.