Mechanisms of renal damage in systemic lupus erythematosus

Autoimmunity and end-organ damage

In considering the pathogenesis of SLE, it is useful to separate autoimmunity from end organ damage. This separation was implicit in the American College of Rheumatology (ACR) Criteria for the Classification of Systemic Lupus Erythematosus. The first nine criteria, that is, malar rash, discoid rash, photosensitivity, oral ulcers, arthritis, sericitis, renal disorder, neurological disorder, and hematological disorder deal with target organ damage while the last two criteria are immunological disorders including positive anti-dsDNA Abs, anti-Sm Abs, anti-phospholipid (aPL) Abs, lupus anticoagulants or false positive serological tests for syphilis and abnormal titers of ANA at any time. The autoimmune parameters are those customarily assayed in clinical laboratories. There have been 180 autoantibodies described for SLE patients. Thus, the absence of ANA and other serological positive tests as specified by the ACR criteria do not rule out SLE since clinical laboratories measure only a small fraction of auto-Abs. The presence of any four of the 11 criteria serially or simultaneously during any interval of observation qualifies the patient to have SLE to be included in clinical studies. It is implicit in these criteria that the laboratory abnormalities may not have direct causal effects on end organ damage. In addition, the ACR criteria for the Classification of SLE should not be used in clinical practice.

Lessons learned from the mouse model NZM2328

Genetic data from the mouse model for LN supports the separation of autoimmunity and end-organ damage. The classic model for LN is the female mice of (NZBXNZW)F1. In order to study the genetics of this model, multiple congenic lines were generated by backcrosses of (NZBXNZW)F1 with NZW and multiple brother and sister mating with coat colors as the selection criteria. These are termed New Zealand Mixed (NZM) strains. The characterization of these strains revealed that one strain has circulating ANA without either kidney disease or neurological disease while other strains have ANA with varying clinical manifestations of kidney disease and/or neurological illness. These phenotypic variations support the hypothesis that autoimmunity as measured by the presence of circulating ANA and end-organ damage are under separate genetic control.

Our genetic studies on NZM2328 provide definitive data to support the above stated thesis. NZM2328 female mice have circulating ANA with IC-mediated nephritis that progresses to end stage renal disease (ESRD) with early mortality while the male mice have positive ANA and IC-mediated nephritis which does not progress to ESRD and early mortality. The pathology of IC-mediated nephritis without progression to ESRD resembles that of acute glomerulonephritis (aGN) while that of IC-mediated nephritis with progression to ESRD is that of chronic GN (cGN) ( Fig. 35.1 A). With ANA/anti-dsDNA Ab as a phenotype for autoimmunity, and aGN and cGN as distinct phenotypes, the analysis of a female cohort of a backcross (NZM2328XC57L/J) X NZM2328 identified that a single locus Cgnz1 on the telomeric end of chromosome 1 was significantly linked to cGN. A locus Agnz1 distinct from Cgnz1 on chromosome 1 was suggestively linked to aGN. Two genetic intervals on chromosome 17, one of which is the H-2–Tnf complex and the other Agnz2 distal to the H-2–Tnf complex, were also suggestively linked to aGN. A single locus Adaz1 on chromosome 4 was suggestively linked to circulating IgG anti-dsDNA Abs.

Two congenic lines, NZM2328.C57L/Jc1 (NZM.C57Lc1) and NZM2328.C57L/Jc4 (NZM.C57Lc4) were generated by replacing the relevant genetic interval containing Cgnz1/Agnz1 or Adaz1 with those from C57J/L as shown in Fig. 35.1 B. NZM.C57Lc1 females had markedly reduced incidence of aGN, cGN, severe proteinuria, and auto-Ab production. NZMC57Lc1 female mice did not have early mortality. NZM.C57Lc4 females developed aGN and cGN. The kinetics of severe proteinuria development in NZM.C57Lc4 is similar to that of the parental line NZM2328. They had early mortality. The development of severe proteinuria without circulating anti-dsDNA Ab was documented ( Fig. 35.1 C and D). These observations led us to postulate an interactive model for the pathogenesis of SLE with autoimmunity and end organ susceptibility to damage under separate genetic control ( Fig. 35.1 E). This model places end-organ, that is, the kidney as an active participant in the pathogenesis of LN. The data in Fig. 35.1 do not support the thesis that anti-dsDNA Abs are of paramount importance in LN. In addition, breaking tolerance to dsDNA and other nuclear antigens are not required for the development of LN.

Genetic data show that aGN, the IC-mediated LN, need not progress to cGN. NZM.C57Lc1 contains the C57L/J genetic interval in which Cgnz1 and Agnz1 loci reside on chromosome 1. Further generation of intrachromosomal recombinant lines separated these two loci. The recombinant strain NZM.c1R27 (R27) in which the Cgnz1 were replaced with the allelic region from C57L/J was informative. As shown in Fig. 35.2 A , R27 female mice at 12 months of age that had mild proteinuria had enlarged glomeruli with normal tubular cells without significant interstitial infiltration. In contrast female NZM2328 mice with severe proteinuria had glomerular sclerosis and marked interstitial infiltration. Despite these differences in kidney pathologies between NZM2328 and R27, the female mice of R27 had comparable IC deposits with complement activation as the parental strain NZM2328 ( Fig. 35.2 A). Thus, female R27 mice were resistant to the development of cGN, severe proteinuria and ESRD. The phenotype of R27 supports the conclusion that IC-mediated acute LN may not progress to cGN, providing evidence that end organ resistance to damage plays a significant role in determining the outcome of IC-mediated LN.

Further generation of intra-chromosomal strains as shown in Fig. 35.2 B provides the genetic region of 1.34Mb in which Cgnz1 resides (the red box in Fig. 35.2 B). The characterization of these recombinant strains reveals the complexity of the c1 region. As shown in Fig. 35.2 C, >85% of the female mice in R314-1, R503-10, R286-7 and R290-2 have severe proteinuria. The incidence of severe proteinuria is more than that in the parental line. This suggests that certain gene(s) in NZM2328 within the region of 169.22-172.16 Mb on chromosome 1 may have suppressive function. Similarly, there appears to be a suppressor gene within 171.14-178.21 Mb on chromosome 1 within NZM2328. Thus, there are significant interactions within the c1 region within NZM.

The finding of resistance genes to end organ damage in the mouse may have translational potential. The human homolog of the genetic region of interest is depicted in Fig. 35.2 D. With the exception of 170000P17Rik the human homolog of Cgnz1 region has identical genes in the exact gene order as that of mouse Cgnz1. This suggests that evolution pressure keeps these genes together.

The complexity of the genetics of the NZM2328 mouse model of LN provides an explanation for the marked variation of the clinical course of human LN. These genetic factors should be taken into consideration of our understanding of the pathogenesis of LN.

Silent LN

Human LN has been considered to be the prototype of IC mediated diseases. The nature of IC deposition has been studied intensively. Clinically IC deposition alone may not lead to declining renal function. Clinically this is exemplified by the natural history of “silent LN.” These patients have little proteinuria and no significant cellular elements in their urine and their renal biopsies showed pathology compatible with LN. It is of interest to note that most patients with proliferative LN (class III and class IV) who had second renal biopsies did not develop further changes in their histopathology. “Silent LN” may have relevance to end organ resistance to damage.