Properdin

Properdin, or factor P, is a single-chain glycoprotein (apparent molecular mass 52-55 kDa), consisting of six thrombospondin repeat sequences between short N- and C-terminal domains (1). In the blood it exists as a mixture of head-to-tail dimers, trimers and tetramers (Fig. 1) in a basic proportion of 26:54:20 (3). This proportion is altered by consumption in vitro, which depletes the oligomeric forms in descending order. Higher oligomers than tetramers seem to be mostly an artifact of properdin purification and handling. The protein is produced by a variety of leukocytes, including monocytes (4), T lymphocytes (5) and neutrophils (6), but also by endothelial cells in which properdin synthesis and release are induced by shear stress (7).

Properdin is one of the six principal proteins that participate in the alternative pathway complement activation, the others being C3 and factors B, D, I and H. It stabilizes the labile C3bBb which is deposited on immune complexes or foreign surfaces. This permits amplification of C3bBb formation in competition with catabolism of C3b by factor I, which uses factor H as a cofactor. This local amplification process leads to the creation of the alternative pathway C5 convertase C3bBb3b and initiates the terminal pathway of complement activation.

Properdin prolongs the half-life of surface-bound C3bBb from 1½ minutes to about 18 minutes. This is due to several effects: inhibition of C3b cleavage by factor I, increased affinity for factor B and inhibition of the dissociation of C3bBb into C3b and Bb. Properdin is consumed by binding to C3bBb, this binding showing an order of preference of tetramers over trimers over dimers, which is also the order of functional activity of the oligomeric forms (3). Deficiency or malfunction of the molecule may lead to severe impairment of alternative pathway activation, depending on the precise nature of the defect. One parameter of functional defect in the presence of measurable levels of protein is an impaired generation of the more active tetramer and trimer forms.

Properdin deficiency

The properdin gene is located on the short arm of the X chromosome (8), and congenital properdin deficiencies are therefore inherited as typical X-linked recessive traits. Three types of familial deficiency have been described: type 1 (or I) is characterized by serum with very low or absent properdin activity in hemolytic assays and <0.1 µg/ml immunoreactive protein (9); type 2 (or II) is characterized by low but detectable levels of immunoreactive protein (»2 µg/ml) and impairment of some, but not all functional tests (9,10); and type 3 (or III) has normal levels of immunoreactive but dysfunctional protein (11). Normal serum or plasma levels of properdin are variously quoted as about 5 or 25 µg/ml. These figures come from different laboratories and the absence of accepted standard preparations means that there is as yet no general consensus about the normal range e.g. for a Caucasian population. Female carriers show "Lyonization", i.e. the random suppression of one or other X chromosome in somatic cells, so that they may have low, medium or near-normal levels of immunoreactive properdin.

Different point mutations have been identified as being responsible for the different types of properdin deficiency. Type-1 deficiencies are due to mutations giving rise to premature stop codons in early exons (12,13,14) or to conserved amino-acid substitutions in later exons, thought to impair secretion and promote intracellular catabolism of the molecule (14). Type-2 deficiencies are due to amino-acid substitutions that appear to be associated with defective oligomerization and increased extracellular breakdown (15). Type-3 deficiency in one family was associated with a Tyr®Asp substitution at position 387 which did not affect oligomerization but impaired function, presumably by affecting binding to C3b (16).

Properdin deficiency may also be secondary to consumption, which can occur as a result of factor H deficiency, a rare autosomal recessive trait (17). This leads to spontaneous in vivo activation of the alternative pathway, with consumption of C3, factor B and terminal pathway components as well as properdin.

Clinical correlates

Properdin deficiency of whatever type is above all associated with enhanced susceptibility to meningococcal disease. The occurrence of fulminant meningococcal infections in three members of a family was in fact what led to the first description of properdin deficiency (18), and this has been the pattern for many subsequent studies. These are not conventional examples of case clustering within families because of close contact: the individual infections are dispersed in time and may be due to uncommon serogroups. However, groups of patients with unselected meningococcal disease show only modest percentages of complement deficiency, e.g. 3% in a Dutch study (19), 11% in an Israeli study (20). The frequency of heritable complement deficiencies, including properdin deficiency, in unselected meningococcal disease may vary with ethnicity: the Israeli study showed a high frequency, with properdin deficiency accounting for 2 of the 11 cases of complement deficiency.

The frequency of complement deficiencies in general and properdin deficiency in particular rises markedly when cases of meningococcal disease are selected for unusual features. A Danish study found complement deficiencies in 14% of cases in which there were two or more infections within a family at an interval of >30 days, in 41% of cases that had suffered an additional episode of meningococcal infection or purulent meningitis of other etiology, and in 19% of cases due to uncommon, presumably low-virulent serogroups such as W-135, 29E, X and Y (21). Taken together these cases showed a predominance of defects of the initiation pathways, properdin deficiency being the most common. In the Dutch study, 33% of patients with meningococcal infections due to uncommon serogroups had complement deficiency, the vast majority being over 5 years of age. 27% of complement-deficient relatives found by screening had had meningococcal disease, this figure being 18% in properdin-deficient (male) relatives.

The fact that only a proportion (i.e. around 20%) of properdin-deficient males actually get meningococcal disease raises the question of which additional factors might affect susceptibility. Chance exposure presumably plays a part, but concurrent immune defects may also be involved. In a Swiss family only 2 of 9 properdin-deficient males had meningococcal disease. These were distinguished from the others by lack of the IgG2m(n) allotype marker (13). In a Danish family 3 of 6 properdin-deficient males had had meningococcal meningitis. There is now as yet unpublished evidence from Denmark that concurrent MBL deficiency may be an important factor in such cases. MBL deficiency, inherited as an autosomal dominant or co-dominant, is itself a risk factor for meningococcal disease (22), as discussed in previous issue of this newsletter.

Screening for properdin deficiency

From the above considerations, it would seem prudent to screen all patients and their blood relatives for properdin deficiency in cases of recurrent meningococcal infection, recurrent bacterial meningitis, time-separated clustering of meningococcal infections within a family, and meningococcal infections due to unusual serogroups. This screening will identify both affected males and a high proportion of the female carriers, whose children should of course also be screened. Deficient individuals should then be considered for vaccination against meningococcal disease. It is to be expected that widespread screening on these indications will show that properdin deficiency is not quite as rare as has hitherto been thought.

It would also seem prudent to screen these families for MBL deficiency, as concurrent properdin and MBL deficiency, the latter being very common, would be expected to multiply the risk of meningococcal disease and heighten the indication for vaccination.

Properdin as a target for anti-inflammatory therapy

As a final note, properdin has also been seen as a target for anti-inflammatory intervention with therapeutic antibodies e.g. after cardiac surgery. It will be apparent from the above that such intervention, like other immunosuppressive interventions, will not be without certain risks. The future will tell just how favorably the benefit/risk equation works out in these situations.

L.O. Uttenthal

(Figure legend)

Fig. 1. Schematic representation of properdin molecules, based on electron micrographs in ref. (2) and showing head (red)-to-tail (green) dimers, trimers and tetramers. The thrombospondin repeat sequences are represented in blue.

References

1. Nolan KF, Schwaeble W, Kaluz S, Dierich MP, Reid KBM (1991) Molecular cloning of the cDNA coding for properdin, a positive regulator of the alternative pathway of human complement. Eur J Immunol 21:771-776.

2. Smith CA, Pangburn MK, Vogel CW, Muller-Eberhard HJ (1984) Molecular architecture of human properdin, a positive regulator of the alternative pathway of complement. J Biol Chem 259:4582-4588.

3. Pangburn MK (1989) Analysis of the natural polymeric forms of human properdin and their functions in complement activation. J Immunol 142:202-207.

4. Whaley K (1980) Biosynthesis of the complement components and the regulatory proteins of the alternative complement pathway by human peripheral blood monocytes. J Exp Med 151:501-516.

5. Schwaeble W, Dippold WG, Schafer MK, Pohla H, Jonas D, Luttig B, Weihe E, Huemer HP, Dierich MP, Reid KBM (1993) Properdin, a positive regulator of complement activation, is expressed in human T cell lines and peripheral blood T cells. J Immunol 151:2521-2528.

6. Wirthmueller U, Dewald B, Thelen M, Schafer MK, Stover C, Whaley K, North J, Eggleton P, Reid KB, Schwaeble WJ (1997) Properdin, a positive regulator of complement activation, is released from secondary granules of stimulated peripheral blood neutrophils. J Immunol 158:4444-4451.

7. Bongrazio M, Pries AR, Zakrzewicz A (2003) The endothelium as physiological source of properdin: role of wall shear stress. Mol Immunol 39:669-675.

8. Goundis D, Holt SM, Boyd Y, Reid KBM (1989) Localization of the properdin structural locus to Xp11.23-Xp21.1. Genomics 5:56-60.

9. Sjöholm AG, Söderström C, Nilsson LA (1988) A second variant of properdin deficiency: the detection of properdin at low concentrations in affected males. Complement 5:130-140.

10. Nielsen HE, Koch C (1987) Congenital properdin deficiency and meningococcal infection. Clin Immunol Immunopathol 44:134-139.

11. Sjöholm AG, Kuijper EJ, Tijssen CC, Jansz A, Bol P, Spanjaard L, Zanen HC (1988) Dysfunctional properdin in a Dutch family with meningococcal disease. N Engl J Med 319:33-37.

12. Westberg J, Fredrikson GN, Truedsson L, Sjoholm AG, Uhlen M (1995) Sequence-based analysis of properdin deficiency: identification of point mutations in two phenotypic forms of an X-linked immunodeficiency. Genomics 29:1-8.

13. Spath PJ, Sjoholm AG, Fredrikson GN, Misiano G, Scherz R, Schaad UB, Uhring-Lambert B, Hauptmann G, Westberg J, Uhlen M, Wadelius C, Truedsson L (1999) Properdin deficiency in a large Swiss family: identification of a stop codon in the properdin gene, and association of meningococcal disease with lack of the IgG2 allotype marker G2m(n). Clin Exp Immunol 118:278-284.

14. van den Bogaard R, FijenCA, Schipper MG, de Galan L, Kuijper EJ, Mannens MM (2000) Molecular characterisation of 10 Dutch properdin type I deficient families: mutation analysis and X-inactivation studies. Eur J Hum Genet 8:513-518.

15. Fredrikson GN, Gullstrand B, Westberg J, Sjoholm AG, Uhlen M, Truedsson L (1998) Expression of properdin in complete and incomplete deficiency: normal in vitro synthesis by monocytes in two cases with properdin deficiency type II due to distinct mutations. J Clin Immunol 18:272-282.

16. Fredrikson GN, Westberg J, Kuijper EJ, Tijssen CC, Sjoholm AG, Uhlen M, Truedsson L. (1996) Molecular characterization of properdin deficiency type III: dysfunction produced by a single point mutation in exon 9 of the structural gene causing a tyrosine to aspartic acid interchange. J Immunol 157:3666-3671.

17. Nielsen HE, Christensen KC, Koch C, Thomsen BS, HeegaardNH, Tranum-Jensen J (1989) Hereditary, complete deficiency of complement factor H associated with recurrent meningococcal disease. Scand J Immunol 30:711-718.

18. Sjöholm AG, Braconier JH, Söderström C (1982) Properdin deficiency in a family with fulminant meningococcal infections. Clin Exp Immunol 50:291-297.

19. Fijen CA, Kuijper EJ, te Bulte MT, Daha MR, Dankert J (1999) Assessment of complement deficiency in patients with meningococcal disease in The Netherlands. Clin Infect Dis 28:98-105.

20. Schlesinger M, Nave Z, Levy Y, Slater PE, Fishelson Z (1990) Prevalence of hereditary properdin, C7 and C8 deficiencies in patients with meningococcal infections. Clin Exp Immunol 81:423-427.

21. Nielsen HE, Koch C, Magnussen P, Lind I (1989) Complement deficiencies in selected groups of patients with meningococcal disease. Scand J Infect Dis 21:389-396.

22. Bax WA, Cuysenaer OJJ, Bartelink AKM, Aerts PC, Ezekowitz AB, van Dijk H (1999) Association of familial deficiency of mannose-binding lectin and meningococcal disease. Lancet 354:1094-1095.