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3.5. Clearance and metabolism

Chapter 3

Serum concentrations of FLCs and intact immunoglobulins reflect the balance between their production and clearance rates. An understanding of immunoglobulin clearance mechanisms in both normal and pathological conditions is important when considering the utility of sFLCs and intact immunoglobulins as tumour markers in monoclonal gammopathies.

3.5.1. Half-life of sFLCs

sFLCs are rapidly cleared and metabolised by the kidneys. At around 25 kDa in size, monomeric FLCs, characteristically κ, are cleared in 2 - 4 hours at 40% of the glomerular filtration rate. Dimeric FLCs of around 50 kDa, typically λ, are cleared in 3 - 6 hours at 20% of the glomerular filtration rate (Figure 3.10), while larger polymers are cleared more slowly [93][94]. In contrast, IgG has a half-life of approximately 21 days with minimal renal clearance (Section 5.3). Although κ FLC production rates are estimated to be twice that of λ, their faster removal ensures that actual serum concentrations are approximately 50% lower (Chapter 5). The half-life of FLCs is dependent upon kidney function, so that FLC removal may be prolonged to 2 - 3 days in MM patients with complete renal failure [91][93][94][95]. In patients with chronic kidney disease (CKD), κ and λ sFLC concentrations increase due to reduced renal clearance [96]. When renal clearance is reduced, a greater proportion of sFLC are removed through pinocytosis by cells of the reticuloendothelial system [97]. This mechanism removes κ and λ sFLC at the same rate so the relative FLC concentrations change to reflect more closely the higher rate of κ production and there are minor increases in the κ/λ sFLC ratio [96].

3.5.2. Renal clearance of FLCs

Figure 3.11 shows the glomerular filtration and metabolism of FLCs within a kidney nephron. Each nephron contains a glomerulus with basement membrane fenestrations, which allow filtration of serum molecules into the proximal tubules. Pore sizes are variable, with restricted filtration of molecules that are greater than 20 kDa in size, and a molecular weight cut-off of around 60 kDa. Protein molecules that pass through the glomerular pores are bound by the multi-ligand megalin and cubulin receptors on proximal tubule epithelium; these are then absorbed unchanged, degraded in the proximal tubular cells into their constituent amino acids, or excreted as fragments [99]. This megalin/cubulin absorption pathway is designed to prevent loss of large amounts of proteins and peptides into urine. It is very efficient and can process between 10 and 30 g of small molecular weight proteins daily. Therefore, the 500 mg of FLCs produced each day by the normal lymphoid system are filtered by the glomeruli and completely processed in the proximal tubules [95][100][101].

In normal individuals, between 1 and 10 mg of FLCs are excreted per day into the urine. Their exact origin is unclear, but they probably enter the urine via the mucosal surfaces of the distal part of the nephrons and the urethra, alongside secretory IgA. This secretion is part of the mucosal defence system that prevents infectious agents entering the body.

Because of the huge metabolic capacity of the proximal tubule, the amount of FLCs in urine (even when production is considerably increased in a patient with MM), is more dependent upon renal function than synthesis by the tumour. As a consequence, serum and urine FLC concentrations may differ during the evolution of light chain MM (LCMM) (Figure 3.12). From low initial starting concentrations, sFLCs increase steadily with growing tumour mass, while concentrations in the urine show little change until the proximal tubular metabolism is exceeded and overflow proteinuria develops. Hence, early disease and oligo-secretory disease are not identified from urine tests. Subsequently, urine FLCs rise rapidly as overflow occurs, to reach a maximum. Concentrations then decrease as renal impairment occurs, and are low in complete renal failure. By contrast, sFLC levels increase as renal impairment develops due to the lengthening half-life of FLCs that are no longer cleared by the kidneys. Because of the biphasic urine curve, decreasing concentrations may indicate response to treatment or deterioration of renal function. Urine measurements are therefore unreliable during disease monitoring. Serum levels, however, rise or fall in correct relationship to worsening or improving disease status. The merits of serum over urine testing are further discussed in Chapter 24.

3.5.3. Half-life of IgG, IgA and IgM

Under normal circumstances, most serum proteins that are too large for renal filtration (> around 60 kDa) are removed by pinocytosis, a process that occurs in all nucleated cells as they obtain their essential nutrients from plasma. This accounts for the half-life of IgA and IgM, which is constant at around 5 - 6 days. By contrast, IgG has a concentration-dependent half-life of approximately 21 days due to recycling by FcRn receptors [103][104][105][102]. These receptors have a structure similar to Class I MHC molecules with a heavy chain of three domains and a single domain light chain comprising β2-microglobulin (Figure 3.13). FcRn receptors are functional in most nucleated cells, including renal podocytes, which may account for the presence of IgG in the urine at high serum concentrations (Chapter 24) [105][106][107][108][109][110][111]. These are the same receptors that transport IgG from the pregnant mother to the developing foetus in the last trimester of pregnancy.

The heterodimeric FcRn molecules protect both IgG and albumin from acid digestion in lysosomes, recycling them back to the cell surface (Figure 3.14). Interestingly, IgG and albumin molecules do not compete for the same sites on the receptor, although the exact mechanism and sites of binding are unknown. In the absence of functioning FcRn receptors, as in patients with familial hypercatabolic hypoproteinaemia (a disease associated with a genetic deficiency of β2-microglobulin), the half-lives of IgG and albumin are only 3 days. Such patients have hypogammaglobulinaemia, not from failure of production, but simply from excessive catabolism.

At high IgG concentrations, the FcRn recycling system can reach saturation and the half-life of IgG falls as there are insufficient FcRn receptors to protect all IgG molecules (Figure 3.15). Hence, a patient presenting with, for example, a monoclonal IgG of 90 g/L is producing far more than 3 times the amount of IgG than a patient presenting with 30 g/L of IgG. In contrast, at low IgG concentrations, when FcRn receptor protection is maximal, the IgG half-life extends to several months. Serum IgG concentrations may therefore be an unreliable indicator of tumour production rates in patients with IgG MM (Chapter 18).

Questions

  1. What accounts for immunoglobulin light chain heterogeneity?
  2. What are the normal serum half-lives of IgG and FLCs?
  3. Do urine FLC concentrations always increase alongside rising sFLC concentrations?
  4. Why does the IgG half-life vary with concentration?

Answers

  1. Light chain heterogeneity arises from genetic recombination, isotypic, allotypic and idiotypic variation and somatic hypermutation of the variable regions after antigen exposure (Sections 3.1 and 3.2)
  2. IgG is approximately 21 days and FLCs 2 - 6 hours (Section 3.4).
  3. No. If there is significant renal impairment, urine FLC excretion falls (Section 3.4).
  4. FcRn receptors saturate at high IgG concentrations so the half-life shortens (Section 3.4). At low IgG concentrations, the half-life lengthens because FcRn receptor recycling is maximal (Section 3.4).
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