A song of AAs and fire: divergent sex-dependent renal inflammatory mechanisms in hypertensive SLE mice

Hypertension remains a pervasive disease throughout the world and is the primary risk factor for cardiovascular (CVD) and renal disease. Patients with systemic lupus erythematosus (SLE), an autoimmune disease, exhibit a three- to fourfold increased incidence of hypertension compared with the general population, which heightens their risk of developing CVD and renal disease (1). Since this patient population is at increased risk of morbidity and mortality, there is a clear need for further research in prediction models, early diagnostics, and targeted therapies for SLE and related cardiovascular and renal dysfunction. Though the underlying pathophysiological mechanisms are still unclear, SLE-associated hypertension and renal dysfunction are generally hypothesized to be a product of immunological intolerance due to a myriad of factors including genetic, humoral, and environmental. SLE is characterized by the production of autoantibodies (AAs), increased circulating proinflammatory cytokines, and overactivation of the T and B cells, which are all posited to lead to end-organ damage. SLE is more prevalent among females, with estimates ranging from a 4.3 to 13.6 times higher prevalence compared with males (2). Thus, many preclinical and clinical studies focus on females, leaving sex differences in the pathogenesis of SLE an understudied area. In a recent article in the American Journal of Physiology-Heart and Circulation, Chaudhari et al. (3) investigated the sex differences in hypertension and renal dysfunction progression in the NZBWF1 mouse model of SLE. The group presents several important findings that begin to fill this critical gap in the literature and which open several avenues for future research.

SEX DIFFERENCES IN ARTERIAL PRESSURE AND RENAL FUNCTION IN SLE MICE

In this latest study (3), mean arterial pressures at 34 to 35 wk of age were ∼30 mmHg higher in the male and female SLE cohorts compared with age- and sex-matched controls. Interestingly, the authors failed to detect any differences in the arterial pressure of male and female SLE mice. Despite the similarities in the hypertensive profiles, female SLE mice exhibited greater renal injury and signaling of the proinflammatory cytokine TNFα compared with the male SLE cohort. Therefore, the authors concluded that the hypertension in the male SLE mice was largely independent of renal injury and dysfunction. In contrast, the group hypothesized that the female SLE hypertension was mediated through a renovascular mechanism because of the coinciding decrease in glomerular filtration rate and increase in renal inflammation/injury and renal vascular resistance compared with their male counterparts. However, the authors did not present a temporal profile of renal function/injury and blood pressure in the current study (3). Therefore, it is difficult to draw conclusions about whether the hypertension in the female SLE mice was due to a greater susceptibility to pressure-dependent renal damage than in male SLE mice. To interrogate this important mechanism, the investigators are encouraged to perform repeated measurements of blood pressure and renal function over time. Moreover, the authors are also encouraged to use an adaptation of methods employed by Evans et al. (4), wherein a servo-controlled cuff mitigated the hypertensive pressure in one kidney. This would allow the researchers to decipher the contributions of arterial pressure and humoral milieu to the renal inflammation and dysfunction in SLE-associated hypertension.

It is also important to note that the lack of pressure differences in male and female SLE mice is an intriguing and unique observation, as there are limited data on preclinical hypertension models with similar blood pressures between males and females in SLE and hypertension research at large. Since premenopausal female models of hypertension often have a reduced hypertensive profile compared with male counterparts (5), the interpretation and mechanistic interrogation responsible for sex differences are often limited.

SEX DIFFERENCES IN RENAL INFLAMMATORY SIGNALING AND AUTOIMMUNE DISEASE IN SLE MICE

The finding that renal TNFα and TLR7 expression are greater in female than male SLE mice is fascinating and offers a potential mechanism for female-specific renal inflammation and injury. This TNFα mechanism has been previously interrogated in female SLE mice (6), where hypertension and renal injury/dysfunction in female SLE mice was attenuated after 4-wk treatment with TNFα antagonist etanercept. Interestingly, there remain opposing hypotheses on the role of TNFα antagonists and agonists in the prevention and treatment of SLE cardiovascular and renal dysfunction. Data from the current study (3) and others (6) collectively support the hypothesis that increased TNFα and downstream inflammatory signaling contribute to renal and cardiovascular disease progression in female SLE mice. In contrast, another group reported that chronic administration (4 mo) of recombinant TNFα (10 µg ip) in the same female mouse model of SLE mitigated the progression of renal injury and dysfunction (7). This study, however, did not report TNFα receptor expression, and these studies should be repeated in light of the recent work presented by Chaudhari et al. (3) and others (6). Because of these conflicting results, it remains unclear whether TNFα signaling plays a causative or associative role in the development of SLE, and further investigation of this relationship would be an excellent avenue for research in a follow-up study. Such follow-up studies would be further bolstered by performing a multiplex cytokine assay or similar analysis of renal tissue, as this would broaden the understanding of how pro-/anti-inflammatory cytokine signaling differs between male and female SLE mice. Furthermore, this would offer additional avenues for investigation that may provide more promising translational potential than those observed with TNFα.

The current study of Chaudhari et al. (3) also observed female SLE mice have higher circulating dsDNA AAs compared with the male SLE cohorts. This may be interpreted as an exacerbated autoimmune severity in the female SLE versus the male SLE, which would align with female-specific renal dysfunction and injury. The dsDNA AA range reported is quite large, from nearly 0 to 1.5 million activity (in U/mL), which may offer the authors an interesting opportunity to perform various correlative analyses with the current data set and in future studies. Specifically, it may offer further insight into the target of the autoimmune disease if the authors performed a correlative analysis of the plasma dsDNA AAs to the matched measurements of renal injury, inflammation, and blood pressures between and within sexes. Moreover, the distribution in the plasma dsDNA AA levels in the SLE mice (n = 25–29 per sex) highlights an important consideration for future studies, where the dsDNA AA ranges should be provided for the specific subsets used for the cardiovascular and renal measurements. The corresponding dsDNA AA data to the subsets representing the arterial pressure (n = 5 per sex) and renal function (n = 4–9 per sex) are unclear. Providing this corresponding data and/or correlative analysis would enable the readers and investigators to visualize the range of these data and the corresponding CV and renal function.

EXPERIMENTAL LIMITATIONS TO CONSIDER IN FUTURE RESEARCH

Although the article by Chaudhari et al. (3) offers an intriguing interrogation of the male and female SLE mouse hypertension models, there is a major limitation in the scope and methodology that requires further consideration in future studies. The accuracy of the blood pressure readings is extremely important to these studies, as this is a critical piece of the sex difference puzzle that the authors are assembling. The similar blood pressure between the male and female SLE mice allows the authors to isolate this variable in the discussion of the differences in renal function and injury. One lingering concern is the cardiovascular data shown in Table 1 of Chaudhari et al. (3), where there is limited separation of the diastolic and systolic pressures in each group. The most likely explanation is that this is a result of clotting, which is common in this indwelling catheterization technique. This likely also accounts for the large variation in the heart rate data because of artefact in the software’s peak-nadir detection, specifically the SLE male HR ranging from ∼450 beats/min to a supraphysiological ∼1,100 beats/min. While the mean arterial pressure data may represent the true blood pressures, the authors and others in the field are encouraged to carefully reexamine raw tracings to determine whether the blood pressure tracings are at steady state with clear systolic and diastolic limits and whether the cycle detection function accurately marks each beat.

In addition, blood pressure was acutely measured by indwelling carotid catheters, presumably during the daytime (inactive period). As with all preclinical blood pressure research, the authors must consider using implanted telemeters to monitor cardiovascular parameters over time, which represents the gold standard for such measurements. Solely relying on end-point measurements of arterial pressure has clear limitations. Therefore, the investigators are encouraged to perform repeat telemetry recordings in at least a subset of the animals. This would permit the authors to carefully assess differences in 24-h mean arterial pressure, as well as important variations such as circadian rhythm. Moreover, these repeated measurements should also be combined with measurements of renal function and injury over time to address this important temporal relationship. For example, renal inflammatory signaling may be estimated by repeated measurements of urinary cytokines, which was recently demonstrated in another preclinical model of hypertension (8). This methodology provides a simple and easily repeated estimation which would allow the authors to examine the temporal profile of renal cytokines in addition to arterial pressure and renal function.

IMPLICATIONS IN FUTURE SLE HYPERTENSION RESEARCH

This latest study by Chaudhari et al. (3) offers a valuable and rare insight into the sex-dependent cardiovascular and renal phenotypes in the NZBWF1 mouse model of SLE. Indeed, the complex autoimmune and inflammatory signaling underlying female and male SLE pathogenesis are likely as convoluted and expansive as a George R. R. Martin novel series. Nonetheless, Chaudhari and colleagues (3) offer the field an excellent foundation, from which additional studies can further investigate sex differences in the etiology of SLE-related hypertension, as well as the putative TNFα-mediated mechanism behind the female SLE renal and cardiovascular pathophysiology. Finally, the authors are encouraged to leverage the unique (no renal dysfunction or injury detected) hypertensive pathogenesis in the male SLE mouse and assess interventions [e.g., renal denervation (9)] and salt sensitivity (10) that were previously tested by Mathis and colleagues in the female SLE mice.

GRANTS

This work was supported in part by National Institutes of Health Grants R00HL141650 (to C.T.B.) and U24DK126110 Subaward 20765 (to C.T.B.) and American Heart Association Grant 21PRE0000216068 (to M.M.G.).

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

M.M.G. and C.T.B. drafted manuscript; M.M.G., S.A., and C.T.B. edited and revised manuscript; M.M.G., S.A., and C.T.B. approved final version of manuscript.