What happens when the body has no CD55 at all
This page collects a presentation I put together on CHAPLE syndrome — the rare genetic disease that first showed the world what CD55 is actually for. It's the clinical counterpart to the rest of this site: instead of switching CD55 off on purpose with CRISPRoff or ZFPoff, these patients are missing it from birth.
A single gene, two very different outcomes
CD55 sits on chromosome 1. Losing one copy is common and mostly silent; losing both copies causes CHAPLE syndrome.
Heterozygous deletions of CD55 are frequent in the population and generally don't cause disease on their own — the one working copy is enough. CHAPLE syndrome only appears when a person inherits a loss-of-function mutation on both alleles, so neither copy of the gene can make functional protein. Because that requires two rare events to line up in the same person, homozygous CD55 deficiency is extraordinarily rare.
Population screens back this up: of roughly 640 people with IBD-like symptoms, only one carried a heterozygous CD55 deletion; screening ~60,000 exomes in the ExAC database turned up 53 heterozygous carriers and just a single homozygous individual. The low diagnosis rate is likely also a testing-availability problem, not only a true-rarity one — genetic testing for CD55 isn't routine outside specialist centers.
CD55 is the complement system's off-switch
Complement is a cascade of blood proteins that tags and destroys pathogens. Left unchecked, it destroys healthy cells too — which is exactly CD55's job to prevent.
Three separate triggers — antibody complexes (classical pathway), microbial sugars (lectin pathway), and spontaneous low-level activity (alternative pathway) — all converge on cleaving C3, which snowballs into cleaving C5. That produces C3a and C5a (anaphylatoxins that recruit neutrophils and macrophages), C3b (which opsonizes targets for phagocytosis), and C5b, which seeds the membrane attack complex that punches holes in cell membranes. CD55 — also called decay-accelerating factor — sits right in the middle of this and accelerates the decay of the C3 and C5 convertases via factors H and I, so C3b and C4b can't keep converting more C3 and C5.
Cascade stays contained
- C3b/C4b decay is accelerated before the cascade can escalate
- C3 and C5 convertases can't keep converting substrate
- No membrane attack complex forms on healthy cells
- Result: no cell destruction
Cascade runs unchecked
- Anaphylatoxins (C3a, C5a) keep recruiting neutrophils and macrophages
- Opsonization drives phagocytosis of healthy cells
- Membrane attack complexes form where they shouldn't
- Result: inflammation and destruction of healthy tissue
CD55 also keeps T cells in check
CD55 isn't only a complement brake — it's also a cofactor for T cell signaling, which is why losing it disrupts more than one arm of the immune system.
On CD4+ helper T cells, CD55 acts as a costimulatory cofactor: it helps activate the cells and promotes their differentiation into CD4+CD25+ regulatory T cells (Tregs), which in turn produce IL-10 — one of the body's key anti-inflammatory cytokines. Losing CD55 removes this brake too, independent of the complement effects above, which is part of why the CHAPLE syndrome is inflammatory on more than one front.
Three faces of one missing protein
Unchecked complement activation plays out differently depending on where in the body it happens — giving CHAPLE syndrome its three defining features.
Angiopathic thrombosis (and Lymphangiectasia)
The overactive immune system attacks blood vessel walls. Clots form to limit further damage and restrict blood flow — but that same clotting strains the vasculature. Traditional anticlotting medicine largely fails here, because the clots are a downstream symptom of complement activation, not the root problem.
Lymphangiectasia refers to vessel damage and clotting which cause a backup in the lymph nodes. That backup creates swelling and leakage throughout the body, as lymphatic vessels dilate under the extra pressure.
Protein-losing enteropathy
Leaking, swollen intestinal lymphatics let key blood proteins — especially albumin — empty into the intestines and exit the body instead of recirculating. The resulting hypoproteinemia drives the edema and malnutrition seen in patients.
Replacing what's missing, not just treating symptoms
Because the root cause is a single missing protein, treatments aimed only at symptoms — anticlotting medication, for instance — tend to be ineffective. Two strategies work by restoring what CD55 normally does:
Reconstitute CD55 itself
Genetic or protein reconstitution of functional CD55 restores decay-accelerating activity directly and reverses abnormal complement activation.
Block the cascade downstream
A complement-inhibitory therapeutic antibody (such as eculizumab) suppresses C5a production, mimicking CD55's braking effect further down the cascade — even without restoring the protein itself.
What the original cohort showed
CHAPLE syndrome was first described in 2017, in a study built around a small, carefully characterized cohort.
Researchers set out to explain the mechanism behind nonsyndromic primary intestinal lymphangiectasia and protein-losing enteropathy. Because CHAPLE syndrome requires mutations on both CD55 alleles, the founding cohort included just 11 patients, spanning ages 3 to 23 years — young enough to weight genetic over environmental causes. The team combined whole-exome sequencing with Sanger sequencing to confirm the biallelic loss-of-function variants, and ran complement assays before and after CD55 reconstitution to demonstrate the mechanism directly.
One notable finding: the IL-10 deficiency seen with CD55 loss was independent of C5a-driven complement activation — evidence that CD55's T-cell cofactor role and its complement-braking role are genuinely separate mechanisms, both contributing to disease. The team also showed that eculizumab could suppress C5a production in patient cells, supporting it as a treatment route.
As reported in the study: patients' T lymphocytes showed increased complement activation with surface deposition of complement and generation of soluble C5a, and both costimulatory function and cytokine modulation by CD55 were defective — effects reversed by genetic reconstitution of CD55 or treatment with a complement-inhibitory antibody.[1]
From a rare disease to a research question
CHAPLE syndrome is a natural experiment: it shows what happens when CD55 is missing entirely, from birth, in every cell. My PhD project asks a related but different question — what happens when CD55 is switched off deliberately and reversibly, in adult T cells, using epigenetic tools like ZFPoff and CRISPRoff, rather than lost through germline mutation? Reading and modeling the resulting CpG-methylation patterns with Nanopore sequencing and machine learning is the other half of that question. The clinical biology here is the reason CD55 is worth studying at the epigenome level in the first place. It's also worth seeing the opposite extreme: in ovarian cancer, CD55 isn't missing — there's too much of it, and some of it ends up somewhere it shouldn't be.
Sources
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[1]
Ozen A, Comrie WA, Ardy RC, et al., "CD55 Deficiency, Early-Onset Protein-Losing Enteropathy, and Thrombosis" — New England Journal of Medicine, 2017