In the present study, we evaluated the levels of antibodies to synaptopodin, a proline-rich protein associated with actin microfilaments found in the foot processes of podocytes. Previous research has suggested that higher synaptopodin expression in podocytes correlates with a significantly better response to steroid therapy []. Additionally, a study by Wang RX et al. indicated the expression of ANXA1 in podocytes []. In our study, we excluded patients with mutations in genes encoding podocyte proteins and collagen, as well as those with FSGS without nephrotic syndrome and possible secondary forms of FSGS. This allowed us to identify a cohort of patients with primary idiopathic FSGS. We observed a significant increase in the levels of antibodies to both synaptopodin and annexin 1 in adult patients with MCD and FSGS presenting with nephrotic syndrome, compared to those with membranous nephropathy and healthy individuals. However, our results also revealed elevated levels of antibodies in patients with MN. Furthermore, we found no association between the levels of antibodies to synaptopodin and annexin 1 and the response to therapy with corticosteroids and calcineurin inhibitors.
The significant increase in antibody levels observed in patients with MCD and some patients with FSGS leads us to hypothesize antibody-mediated damage to podocytes and potential B-cell dysregulation [11]. However, a counterargument arises from previous published work, similar to the findings regarding antibodies to nephrin. Specifically, the levels of antibodies to other components of podocytes, such as synaptopodin and annexin 1, were not associated with the response to immunosuppressive therapy in patients with podocytopathies [14].
Our findings also revealed a noteworthy trend: levels of synaptopodin antibodies tend to decrease with age, suggesting a potentially less active immune response in adults. Moreover, in patients with more severe TIF, the levels of both antibodies were lower, implying a potential decrease as the disease progresses, particularly at later stages marked by the development of fibrosis in the kidney. Unlike animal models where disease onset can be precisely established, such as [,,], in humans, pinpointing the exact onset of the disease is challenging. Consequently, we cannot entirely rule out the pathological role of anti-podocyte antibodies in the initial damage to podocytes, nor can we disregard the possibility of a decline in antibody levels over time. Severe podocyte damage at disease onset and the subsequent development of podocytopenia may serve as a trigger for EMT and fibrosis processes within the kidney [].
It is intriguing that in podocytopathies, particularly in MCD, the levels of antibodies to various podocyte antigens are elevated. However, the determination of individual antibodies alone does not suffice as a sensitive diagnostic test for identifying primary podocytopathies (MCD and FSGS). Nonetheless, by evaluating the increase in both antibodies to synaptopodin and annexin 1, we can enhance diagnostic accuracy. This approach yields a higher sensitivity of 80.9% and specificity of 81.3%, with an AUC of 0.859 (95% confidence interval [CI] 0.760–0.957) for the diagnosis of podocytopathies (MCD and FSGS). For comparison, aPLA2R exhibits a sensitivity of 65% (63–67%), specificity of 97% (97–98%), and an AUC of 0.939 for diagnosis of MN []. This suggests the potential utility of developing a panel, possibly incorporating other anti-podocyte antibodies, such as those targeting nephrin, to further enhance the diagnostic accuracy of tests for primary podocytopathies. However, this hypothesis requires further validation in larger cohorts.
Similar to antibodies targeting nephrin, the role of antibodies against synaptopodin and annexin 1 in predicting response to immunosuppressive therapy remains unclear. This uncertainty may stem from variations in the duration of the disease prior to antibody testing, as well as differences in the severity of podocytopenia and fibrosis among patients. Moreover, the mechanism by which immune cells are activated to produce various antibodies targeting different molecular structures on the surface of podocytes, recognized by the immune system, remains incompletely understood [11]. The discovery of numerous anti-podocyte antibodies in primary podocytopathies by various research groups raises questions about whether these antibodies are causative agents or mere indicators of damage. Considering the release of numerous antigens/epitopes, both surface and intracellular, during podocyte damage—such as the actin cytoskeleton and slit diaphragm—against which a secondary immune response can develop, antibodies may serve as indicators of podocyte damage. However, the trigger factor for such immune activation remains elusive and requires further research.
One limitation of our study is the small number of patients within each group, which may limit the generalizability of our findings. Additionally, the absence of antibody dynamics over the course of immunosuppressive therapy and variations in the duration of NS prior to antibody assessment are also notable limitations.
In conclusion, our study found significantly higher levels of antibodies to synaptopodin and annexin 1 in patients with podocytopathies, specifically MCD and FSGS, compared to those with MN and healthy individuals. Elevated levels of both anti-annexin 1 and anti-synaptopodin antibodies concurrently exhibited moderate to high sensitivity and specificity for the diagnosis of MCD or FSGS. However, these elevated antibody levels were not associated with the likelihood of achieving remission.