These data show that renal microthrombosis is highly prevalent in LN, affecting half of the patients with proliferative LN. It is more prevalent in patients with aPL, consistent with previous reports [13
]. Moreover, microthrombosis correlates with higher macrophagic infiltration as a marker of inflammatory activity but not with C4d deposition. However, only macrophagic infiltration showed a correlation with SLE renal activity (proteinuria and active sediment), and neither the presence of CD61+
microthrombi nor C4d deposition correlated with LN severity or outcome. Although the mechanisms of thrombosis may be different, previous studies point to complement activation and inflammation as common mechanisms in LN- and aPL-associated vascular pathology. In APS, a "two hits" theory has been proposed, in which the presence of aPL would lead to thrombophilia through endothelial cell, platelet, and monocyte activation ("first hit"), and a "second hit" related to inflammation (that is, Toll-like receptors and complement activation, in response to microbial agents, might synergize and result in clotting events [25
]. Our data link inflammatory infiltration and thrombosis in LN but are less clear regarding the relation of complement activation and thrombosis.
Several studies point to the involvement of complement in the pathogenesis of APS. In animal models, complement factors C3 and C5 are essential in aPL-mediated pathology [15
]. In these models, endothelial cell activation and inflammatory cell infiltration around blood vessels are observed in placental lesions, suggesting that complications of pregnancy are primarily due to inflammation [14
]. In humans, a few studies also point to complement activation in aPL-mediated thrombosis [26
]. Sera from patients with positive aPL enhance complement fixation on platelets, which is associated with arterial thrombosis [16
]. Platelet-associated C4d has been reported in 18% of patients with SLE, particularly in those with aPL [29
]. In addition, hypocomplementemia and increased plasma levels of anaphylotoxins C3a and C4a have been described in primary APS [26
]. However, a direct correlation between increased levels of anaphylotoxins and clinical thrombosis has not been demonstrated [26
Specifically in LN, immune-mediated vascular injury may predispose to platelet accumulation and in situ
complement activation. Intense glomerular C4d deposition has been previously found associated with renal microthrombi in LN patients with or without aPL [15
]. However, because both processes may be associated with proliferative LN and higher activity indexes, this may be an indirect association [26
]. We have specifically analyzed patients with proliferative LN and considered macrophage infiltration as an additional marker of inflammatory activity. Our data do not demonstrate a direct relation between C4d deposition and microthrombosis. Instead, C4d deposition correlated with the intensity of macrophagic infiltration, which in turn was associated with microthrombosis. Deposition of C4d in peritubular capillaries reflected SLE activity, consistent with previous reports [30
]. Neither C4d nor microthrombi were directly related to the severity or outcome of LN.
The detection of CD61 intravascular platelets with IHC is far more sensitive to acute microthrombi in patients with LN compared with routine histologic methods. Thrombi detected with regular histology might have a different impact on renal outcomes but are rarely observed. Clinical observations support that TMA in APS or SLE nephritis has an important impact on renal function and requires specific therapy. However, TMA may occur independent of SLE nephritis, and its prevalence associated with LN is very low. Only two cases had microthrombi detected by histology in our series, precluding analyses of the clinical and subclinical (immunohistochemical) correlations in LN. Our data do not support any difference in renal manifestations and outcomes at 6 and 12 months, and further follow-up after therapy in patients with IHC detected CD61-positive microthrombi. The retrospective design of our study limits the interpretation of results regarding clinical correlations and prognosis. Microthrombi detected with IHC may have a lower impact on renal function and outcome than do larger histologic microthrombi. Longitudinal prospective studies, including patients with LN and repeated biopsies after induction treatment, will help us to better define the real significance of this kind of histologic and IHC lesion.
Macrophagic infiltration was the best marker of renal impairment at kidney biopsy, consistent with previous studies [32
]. Glomerular and interstitial macrophage infiltration is a feature of the more aggressive forms of LN and the individual variable that best correlates with clinical parameters, including SLE activity and renal manifestations. We found a correlation with activity measures, including the level of proteinuria and active sediment, but not with renal function at biopsy or follow-up. A previous study showed that the persistence of glomerular and extraglomerular macrophagic infiltrates after therapy correlates with long-term renal-function outcomes (doubling of serum creatinine and end-stage renal disease) [32
]. However, neither these nor our data demonstrates predictive value for the degree of macrophage infiltration at first biopsy and renal-function outcomes.