2. Results and Discussion
Characteristic and clinical conditions of study animas are shown in
Table 1. The 21 naturally FIV-infected cats were older compared to experimentally infected subjects and controls (median ages statistically difference at
p < 0.01). Fifteen of the naturally infected cats were males and 6 females. Of these, three females and one male were neutered, remaining 17 were intact. Finally, ten of these were in the asymptomatic phase (clinical staging 3), while the other eleven were symptomatic (clinical staging 4). Experimentally FIV-infected cats and controls were all intact females and aged between 2 and 6 years at time of analysis. At time of sacrifice, these subjects were all healthy except one infected with FIV-M2 strain.
Table 1.
Characteristics and creatinine and urine protein concentrations of study cats.
Table 1.
Characteristics and creatinine and urine protein concentrations of study cats.
Characteristic | Naturally Infected (n = 21) | Experimentally Infected | Controls (n = 4) |
---|
FIV-Pet (n = 17) | FIV-M2 (n = 28) | FIV-Pet + FIV-M2 (n = 6) |
---|
Median (Range) | Median (Range) | Median (Range) | Median (Range) | Median (Range) |
---|
Age (years) | 8.0 (4.0–13.0) | 3.0 (2.0–6.0) | 4.0 (2.0–6.0) | 4.5 (4.0–5.0) | 4.5 (4.0–6.0) |
Sex | | | | | |
Male | 15 | | | | |
Female | 6 | 17 | 28 | 6 | |
Neuter status | | | | | |
Intact | 17 | 17 | 28 | 6 | |
Altered | 4 | | | | |
Clinical status | | | | | |
Sick | 11 | | 1 | | |
Healthy | 10 | 17 | 27 | 6 | |
Creatinine concentration | 120 (73–709) | 123 (100–131) | 118 (93–144) | 121 (103–139) | 107 (95–125) |
UPC | 1.17 (0,18–14.00) | 0.69 (0.31–7.00) | 0.52 (0.25–18.00) | 0.92 (0.55–13.00) | 0.25 (0.17–0.31) |
Figure 1 shows the CD4
+/CD8
+ T-cell counts and viral load at the time of sacrifice in the experimentally infected subjects. In these animals a progressive decline of CD4
+ T-cell and a reduced CD4
+/CD8
+ T-cell ratio were observed. This was particularly evident in the subjects sacrificed ≥36 months pi. In contrast, the proviral load in peripheral blood mononuclear cells (PBMCs) constantly increased over time. Although competitive and real-time PCR assays to quantitate the FIV RNA genome yielded different and difficult to compare results, increased levels of plasma viremia relative to time of infection were also observed within FIV-Pet and FIV-M2 groups (data not shown). These results indicated that, notwithstanding the absence of overt clinical symptoms, the FIV infection progressed and eroded immune system efficiency in all infected animals irrespectively of the infecting strains. In fact, no significant differences in lymphocyte subset changes and proviral load were observed between FIV-Pet and FIV-M2 groups. Interestingly, FIV-Pet + FIV-M2 group had CD4
+ and CD8
+ T-cell levels and ratios comparable to animals infected three years earlier with either two viruses alone but the total proviral load in PBMCs,
i.e., FIV-Pet plus FIV-M2 genomes first determined by competitive PCR then reexamined by real-time PCR, was lower compared to FIV-Pet and FIV-M2 groups.
Figure 1.
Analysis of CD4+ and CD8+ T-cell levels and proviral load in the peripheral blood of the examined cats at different times from infection. Solid and empty black circles indicate percent of CD4+ and CD8+ T-cells, respectively. Light blue circles indicate the CD4+ and CD8+ T-cell ratio that was calculated by dividing the CD4+ value by the CD8+ value. Red circles indicate the proviral load expressed as numbers of proviruses per µg of PBMC DNA and determined by competitive or real-time PCRs. The proviral load values for FIV-Pet + FIV-M2 cats are the total number of FIV-Pet and FIV-M2 proviral genomes. Whiskers indicate the standard deviation.
Figure 1.
Analysis of CD4+ and CD8+ T-cell levels and proviral load in the peripheral blood of the examined cats at different times from infection. Solid and empty black circles indicate percent of CD4+ and CD8+ T-cells, respectively. Light blue circles indicate the CD4+ and CD8+ T-cell ratio that was calculated by dividing the CD4+ value by the CD8+ value. Red circles indicate the proviral load expressed as numbers of proviruses per µg of PBMC DNA and determined by competitive or real-time PCRs. The proviral load values for FIV-Pet + FIV-M2 cats are the total number of FIV-Pet and FIV-M2 proviral genomes. Whiskers indicate the standard deviation.
As described in a previous paper [
5], these animals were preinfected with scaled doses of a FIV-Pet that had been cultivated
in vitro and had lost most of its pathogenic potential, and which thus established a low-grade infection in all the inoculated animals. Seven months later animals were challenged with a fully virulent strain of FIV-M2 and monitored for over three years. The results revealed that preinfection with subtype A FIV-Pet did not prevent superinfection and nor did the acute phase of infection give rise to subtype B FIV-M2. However, two years post FIV-M2 inoculation, FIV-Pet preinfection significantly prevented the increase in viral burden compared to control cats infected in parallel with FIV-M2 [
5]. The reduced viral burden observed one year later, when the animals were sacrificed to analyze viral distribution and histopathology in tissues, are thus in line with our follow-up results.
Histopathological examinations of renal tissues showed glomerular changes in 18/21 (85.7%) of the naturally and in 26/51 (51.0%) of the experimentally FIV-infected cats. No alterations were detected in controls (
Table 2).
Mesangial widening, characterized by an increase in mesangial matrix and with or without segmental glomerulosclerosis (
Figure 2A), was observed in 6/17 FIV-Pet infected cats (two 12 months pi, one 24 months pi, and three 36 months pi). One cat infected by 12 months and two cats infected by 24 months with FIV-Pet also showed focal and segmental mesangioproliferative glomerulonephritis (GN) (
Figure 2B). In these areas, IgG (
Figure 2C) and C3 deposits were detected by immunohistochemistry (IHC). In the cats infected with FIV-M2, the renal changes were detected in 12/28 subjects: 6 of these had mesangial widening (two infected by 12 months, one by 24 months, and three by ≥36 months), five were affected by focal and segmental mesangioproliferative GN (one cat each infected by 12 and 24 months, and 3 by ≥36 months). Finally, one cat infected by ≥36 months showed membranoproliferative GN. In this case, the mesangial cellularity had increased and presented enlarged and thickened capillary walls that caused “splitting” of glomerular basement membrane (
Figure 2D). Five of the six cats infected with both strains and sacrificed at ≥36 months pi showed mesangial widening (two cats) and focal and segmental mesangioproliferative GN (three cats).
Six FIV-Pet, eleven FIV-M2, and three FIV-Pet + FIV-M2 infected cats showed degenerative changes in the tubular epithelial cells. Tubular microcysts as well as giant protein casts were occasionally observed. In particular, the tubular mycocyst were detected in four cats, two infected with FIV-M2 and in two with both viruses and giant protein casts in two cats infected with both viruses and sacrificed 36 months pi. Interstitial changes were also infrequent. Scattered periglomerular infiltrates were detected in one FIV-Pet cat sacrificed at 24 months pi, three infected with FIV-M2 (one sacrificed at 24 months and two sacrificed ≥36 months pi) and two cats infected with both viruses.
Table 2.
Renal alterations detected in experimentally feline immunodeficiency virus (FIV)-infected cats sacrificed at the indicated times post-infection (pi).
Table 2.
Renal alterations detected in experimentally feline immunodeficiency virus (FIV)-infected cats sacrificed at the indicated times post-infection (pi).
Renal Alterations | Naturally Infected n = 21 (%) | Controls | 12 Months pi | 24 Months pi | ≥36 Months pi |
---|
FIV-Pet n = 6 (%) | FIV-M2 n = 10 (%) | FIV-Pet n = 7 (%) | FIV-M2 n = 6 (%) | FIV-Pet n = 4 (%) | FIV-M2 n = 12 (%) | FIV-Pet + FIV-M2 n = 6 (%) |
---|
Mesangial widening | 9 (42.9) | 0 | 2 (33.3) | 2 (20.0) | 1 (14.3) | 1 (16.6) | 3 (75.0) | 3 (25.0) | 2 (33.3) |
Glomerulo-nephritis | 3 (14.3) | 0 | 1 (16.6) | 1 (10.0) | 2 (28.6) | 1 (16.6) | 0 | 4 (33.3) | 3 (50.0) |
Glomerular amyloidosis | 8 (30.1) | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
Tubular changes | 10 (47.6) | 0 | 2 (33.3) | 2 (20.0) | 1 (14.3) | 2 (33.3) | 3 (75.0) | 7 (58.3) | 3 (50.0) |
Interstitial lesions | 17 (81–0) | 0 | 0 | 0 | 1 (14.3) | 1 (16.6) | 0 | 2 (16.7) | 2 (33.3) |
Interstitial amyloidosis | 7 (33.3) | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
No alterations | 3 (14.3) | 4 (100) | 3 (50.0) | 7 (70.0) | 4 (57.1) | 4 (66.6) | 1 (25.0) | 5 (41.6) | 1 (16.6) |
Figure 2.
Representative results of renal lesions observed in FIV-M2-infected cats. (A) Mesangial widening. Mild increase in mesangial matrix with a minimal increase in intraglomerula cellularity. Jones’ periodic acid-silver methenamine stain. (Bar = 80 µm); (B) Segmental mesangioproliferative glomerulonephritis. Scattered areas of mild proliferation of mesangial cells with scanty inflammatory infiltrates. Hematoxylin-Eosin stain. (Bar = 80 µm); (C) Mesangioproliferative glomerulonephritis. Segmental deposition of IgG. Strepavidin biotin peroxidise complex method, Mayer’s hematoxylin counterstain. (Bar = 80 µm); (D) Membranoproliferative glomerulonephritis. Mesangial enlargement and thickening of capillary walls with tram track appearance. Azan trichromic stain. (Bar = 80 µm).
Figure 2.
Representative results of renal lesions observed in FIV-M2-infected cats. (A) Mesangial widening. Mild increase in mesangial matrix with a minimal increase in intraglomerula cellularity. Jones’ periodic acid-silver methenamine stain. (Bar = 80 µm); (B) Segmental mesangioproliferative glomerulonephritis. Scattered areas of mild proliferation of mesangial cells with scanty inflammatory infiltrates. Hematoxylin-Eosin stain. (Bar = 80 µm); (C) Mesangioproliferative glomerulonephritis. Segmental deposition of IgG. Strepavidin biotin peroxidise complex method, Mayer’s hematoxylin counterstain. (Bar = 80 µm); (D) Membranoproliferative glomerulonephritis. Mesangial enlargement and thickening of capillary walls with tram track appearance. Azan trichromic stain. (Bar = 80 µm).
Glomerular changes were detected in 18/21 naturally infected subjects. Mesangial matrix with occasional segmental glomerulosclerosis was observed in 9/21 cats (
Figure 3A), in this case protein droplets were frequently detected within podocytes. Immune-mediated GN of mesangioproliferative type in 3/21 cats, and amyloid deposition was present in 8/21 cats (
Figure 3B). Amyloid deposits were segmental and focal in six cases and diffuse in two. In all cases the amyloid deposits were KMnO4 sensitive. Tubulointerstitial alterations were also a common finding in the naturally infected cats: degeneration of tubular epithelial cells was observed in ten cats, tubular microcysts in eight (
Figure 3C), and giant protein tubular casts (
Figure 3D) in four subjects. Interstitial alterations were also frequent and consisted of interstitial infiltration by lymphocytes and plasma cells. Infiltration was scanty periglomerular (eight subjects), diffuse without fibrosis (six), and diffuse with interstitial fibrosis (three). Interstitial amyloidosis was detected in seven subjects, while no interstitial alterations were detected in four cats. Like above, the amyloid deposit was KMnO4 sensitive.
Table 3 summarizes the results of IHC analyses in experimentally and naturally infected cats.
Figure 3.
Representative results of renal lesions observed in naturally FIV-infected cats. (A) Mesangial widening with segmental glomerulosclerosis. Increase in mesangial matrix with a minimal increase in intraglomerular cellularity. Protein droplets were detectable within podocyte cytoplasmas. Azan trichrome stain. (Bar = 80 µm); (B) Glomerular amyloidosis. Diffuse increase of capillary walls due to amyloidosis depostion. Congo red stain. (Bar = 80 µm); (C) Tubular mycrocysts. Presence of interstitial infiltration and dilated tubules forming tubular mycrocysts. Hematoxylin-Eosin stain. (Bar = 80 µm); (D) Giant protein cats. PAS-positive proteinacous cats within tubular microcysts. PAS stain. (Bar = 80 µm).
Figure 3.
Representative results of renal lesions observed in naturally FIV-infected cats. (A) Mesangial widening with segmental glomerulosclerosis. Increase in mesangial matrix with a minimal increase in intraglomerular cellularity. Protein droplets were detectable within podocyte cytoplasmas. Azan trichrome stain. (Bar = 80 µm); (B) Glomerular amyloidosis. Diffuse increase of capillary walls due to amyloidosis depostion. Congo red stain. (Bar = 80 µm); (C) Tubular mycrocysts. Presence of interstitial infiltration and dilated tubules forming tubular mycrocysts. Hematoxylin-Eosin stain. (Bar = 80 µm); (D) Giant protein cats. PAS-positive proteinacous cats within tubular microcysts. PAS stain. (Bar = 80 µm).
Table 3.
Main immunohistochemical findings in naturally and experimentally FIV-infected cats and controls.
Table 3.
Main immunohistochemical findings in naturally and experimentally FIV-infected cats and controls.
Immunohistochemistry | Control Cats n = 4 (%) | Naturally Infected cats n = 21 (%) | Experimentally Infected Cats |
---|
FIV-Pet n = 17 (%) | FIV M2 n = 28 (%) | FIV-Pet + FIV M2 n = 6 (%) |
---|
IgG deposits in mesangium | 0 | 3 (14.3) | 3 (17.6) | 5 (17.9) | 3 (50.0) |
IgG deposits in capillary loops | 0 | 1 (04.8) | 1 (05.9) | 2 (07.1) | 2 (33.3) |
IgM deposits | 0 | 14 (66.7) | 6 (35.3) | 6 (21.3) | 3 (50.0) |
IgA deposits | 0 | 0 | 0 | 0 | 0 |
C3 deposits | 0 | 14 (66.7) | 6 (35.3) | 6 (21.3) | 3 (50.0) |
Mouse monoclonal anti-human AA amyloid | 0 | 8 (38.1) | 0 | 0 | 0 |
Rabbit polyclonal anti-feline AA amyloid | 0 | 8 (38.1) | 0 | 0 | 0 |
Rabbit polyclonal anti-feline AL amyloid | 0 | 0 | 0 | 0 | 0 |
No immunoglobulins or C3 deposits were detected in the uninfected control cats. Both in experimentally and naturally FIV-infected cats, glomeruli affected by GN showed segmental and predominantly mesangial granular deposits of IgG, IgM and C3, while rarely scattered deposits were detected along the capillary loops, IgA staining was not observed. Large proteinaceous casts were positive for IgG and weak for IgA. Amyloid deposits were always positive for the mouse monoclonal anti-human AA and the rabbit polyclonal against the feline AA amyloid, while they were always negative for the rabbit polyclonal anti-AL amyloid.
Mesangial widening, GNs and tubular alterations were detected both in experimentally and naturally infected cats. Giant protein tubular casts and tubular microcysts were more frequently detected in naturally than experimentally infected subjects (p < 0.05). Interstitial alterations were also more frequent in naturally compared to experimentally infected cats (p < 0.001). Further, the former group presented glomerular and interstitial amyloid deposits that were not detected in the experimentally infected ones (p < 0.001). It should be mentioned, however, that a few naturally FIV-infected subjects were old and part of these renal changes, in particular the interstitial ones, could be aged related.
Similarly to previous studies [
6,
7], these results demonstrate that the experimentally FIV infected cats had renal changes similar, to some extent, to those detected in natural infection, and that infected animals exhibit significantly higher rates of renal dysfunction and histological changes in FIV-infected compared to age-matched, FIV-seronegative animals. Examinations of 326 sick cats from Australia demonstrated a significant association between FIV infection and azotemia and palpably small kidneys [
8]. Small kidneys were also reported by Brown and colleagues [
9]. Nonspecific renal abnormalities have also been found in other studies [
10,
11]. Renal alterations in FIV infected cats were observed 5.5% in cats from New Zealand [
12], 9.3% in Japan (from a survey of 700 cats) [
13], and 9% in 76 cats from three Italian regions (Piedmont, Liguria and Val d’Aosta) [
14].
In experimentally FIV-infected cats, which were specific pathogen-free, maintained in isolation units, and regularly checked for various clinical and pathological conditions as well as various pathogens, the main alterations observed were mesangial widening with or without segmental glomerulosclerosis and immune-mediated GNs. These renal changes were also observed in naturally FIV-infected subjects though renal amyloidosis and the presence of interstitial infiltrates seemed to occur only in this latter group. Immune-mediated GNs were observed in 12/51 experimentally and in 3/21 naturally FIV-infected cats. Although the incidence of these immune-mediated alterations seems higher in doubly infected animals, the numbers are too small to draw any certain conclusion. The incidence however does not appear to be related to the infecting strain.
Although FIV-infected cats often present hypergammaglobulinemia, which is believed to be triggered by chronic polyclonal B-cell activation [
15] and consequent production of immune complexes [
15,
16], immune-mediated GNs are no frequently reported in FIV infection. In a previous study on 15 naturally FIV-infected cats only one subject showed IgG deposits in mesangial areas [
6].
Mesangial widening with or without segmental glomerulosclerosis was also detected in experimentally and naturally FIV-infected cats. These alterations [
6] as well as nephrosclerosis [
11] and thickened Bowman’s membrane [
9] have been already reported in natural FIV infections. Such damage is caused by glomerular reactions, which have also been observed in many other, apparently unrelated, clinical entities. These alterations are thus thought to result from intraglomerular hemodynamic alterations [
17]. Hemodynamic alterations in FIV infection might be triggered by a sustained production of lymphokines and/or other host and/or viral factors that stimulate mesangial proliferation, and alter glomerular capillary permeability. Although controversial, there is increasing evidence for a direct viral role on renal cells either as the result of exposure to viral proteins or direct infection renal parenchyma [
7]. On the other hand, tubular and interstitial lesions due to lymphocytes and plasma cell interstitial infiltration, fibrosis and tubular degenerative changes have been mainly detected in naturally FIV-infected cats [
6,
9,
11,
18] and only occasionally in experimentally infected animals. Our study also confirmed previous observations showing glomerular and interstitial amyloidosis in the kidneys of naturally infected animals [
7,
18,
19]. In contrast, none of the 51 experimentally infected cats examined had amyloid deposits in the kidneys. Histochemical and immunohistochemical studies demonstrated that the amyloid deposits were related to secondary amyloidosis, which is typically associated with chronic infections. As a further contribution to the pathogenetic mechanisms of renal disease under FIV infection and seeing that the experimentally FIV-infected cats had no amyloid deposition, the present study would seem to demonstrate that FIV infection alone is not sufficient.
Clinical and pathological studies evidenced that mild to severe renal proteinuria without clinical signs of azotemia is more likely during FIV infection [
6]. In a recent study, performed among client-owned cats, no association was detected between FIV infection and renal azotemia was found but 25% (16/64) of naturally FIV-infected cats were proteinuris compared to FIV-uninfected cats (10.3%, 20/195) [
20]. Also in our study, only 1/51 experimentally infected animal by FIV-M2, sacrificed one year pi, showed azotemia (serum creatinine 144 µmol/L and mild proteinuria 3.9 g/L with urine protein concentration (UPC) 0.43) (Chronic Renal Failure stage 2) [
21]. Renal proteinuria, diagnosed by UPC, was present in 10/17 (56.5%), 9/28 (32.1%), and 2/6 (33.3%) cats infected with FIV-Pet, FIV-M2, and FIV-Pet + FIV-M2, respectively and regardless of the time from infection. However, the most severe renal proteinuria was observed in the FIV-M2 group (mean 3.27 ± 6.34; ranged 0.53–17.63), it was milder in the FIV-Pet + FIV-M2 (mean 3.0 ± 4.39; ranged 0.55–13.01), and was the least severe in the FIV-Pet group (mean 2.56 ± 3.03; ranged 0.31–7.02). A total of 14/29 cats with no histological renal alterations had the lowest mean uP value (3.95 ± 1.37 g/L (2.5–7.05)) and the lowest mean UPC value (0.45 ± 0.12 (0.25–0.67)). Mean uP and UPC values in 10 proteinuric cats with mesangial widening were slightly higher 3.54 ± 1.52 g/L (2.0–5.4) and 0.81 ± 0.44 (0.53–1.94), respectively. Five cats with similar mesangial alteration were aproteinuric (>2.0 g/L). Significantly higher mean uP (21.53 ± 29.39 g/L (2.8–100.0)) and UPC values (4.8 ± 5.77 (0.90–17.63)) were found in 12 cats with glomerular alterations. All 20 cats with tubular alterations had either GN (9/20) or mensangial widening (11/20). Three of them were aproteinuric. Finally, 4/6 cats with interstitial infiltrates were proteinuric, two were infected by 24 months and two by ≥36 months. Electrophoresis of urine proteins confirmed the correlation between proteins excreted in the urine and the histological alterations found in the observed cats: 3/21 (14.3%) cats had glomerulo selective proteinuria, 15/21 (71.5%) had glomerulo non-selective, and 3/21 (14.3%) manifested glomerulo non-selective and tubular proteinuria.
Most experimentally infected cats (71.4 to 100%) had inverted CD4
+/CD8
+ T-cell ratio that depended on the infecting viral isolate and, albeit with low or no statistical significance, time of infection. No correlation between CD4
+/CD8
+ T-cell ratio and renal alteration was found. Likewise, CD4
+/CD8
+ T-cell ratio and CD4
+ T-cell counts did not appear to be related with azotemia and proteinuria. This seems to be in agreement with what has been observed in HIV positive patients [
9].
Our findings indicate that FIV infection can lead to nephropathy. At glomerular level mesangial cell hyperplasia, focal/segmental glomeruloscerosis, global glomerulosclerosis, reactive visceral epithelium, “collapse” capillaries and dilated Bowman’s space have been described as being associated with FIV and HIV infections as well as foot process effacement, wrinkling of glomerular basement membrane, endothelial tubulareticular inclusions, tubular and interstitial nuclear bodies and chromatin degeneration by electron microscopy [
6,
22]. This would suggest that FIV and its natural host could be a useful animal model to investigate lentiviral-induced renal diseases.
Autopsy and biopsy series demonstrated that GNs are present in 10–80% HIV infected patients with renal disease [
22,
23,
24,
25,
26]. These GNs take manifest in various histologic forms including proliferative, lupus-like, and mixed proliferative or sclerotic forms [
23]. Other types of GNs, including the membranoproliferative, have also been reported in HIV infection [
23]. These renal alterations, however, have not been firmly linked to HIV infection: in fact, they may be consequence of coexisting infections [
27,
28,
29], deranged immune response response to coexisting infections [
30], or simply coincidence [
24]. It has been demonstrated that the immune response to HIV infection could culminate in specific HIV-immune-mediated GN [
31]. Our study, which was carried out with cats experimentally infected by FIV and kept in isolation units confirmed that similar immune-mediate alterations can take place and complicate lentiviral infections.
Another renal disorder believed to be associated with HIV infection is HIV-associated nephropathy (HIVAN). HIVAN patients may develop a spectrum of renal pathologies that are typically manifested with an acute and rapid loss of renal function, along with a presence of proteinuria, nephrotic syndrome, and azotemia [
32]. The pathognomonic pathologic findings are characterized by focal segmental glomerulosclerosis, including collapsing glomerulopathy [
33,
34].
Although the pathogenetic mechanisms of HIVAN are mostly unknown, there is evidence to support a causative role of the lentiviral infection. In fact, HIVAN can be reproduced in HIV-1 positive transgenic mice and rats, and non-human primates [
35,
36,
37,
38]. In fact, under highly active antiretroviral therapy some patients experienced reversal renal histologic and laboratory abnormalities as demonstrated in small clinical studies of patients with biopsy-proven HIVAN [
39,
40]. Interestingly, glomerulosclerosis, tubulointerstitial disease and/or mesangial widening with IgM and C3 and mild IgG deposition have also been observed in naturally FIV-infected cats [
6] and, as demonstrated by the present study, also in experimentally conditions though in a smaller proportion of infected subjects than in naturally infected cats.
Studies carried out on renal tissues of HIV-infected patients have demonstrated the presence of HIV DNA [
23]. The recent identification of HIV-1 messenger RNAs in renal tissues and particularly in glomerular and tubular epithelial cells supported the idea that HIV can replicate in this body compartment [
41,
42,
43]. The hypothesis that FIV may play a role in the pathogenesis of renal alterations is supported by the presence of p24 viral antigen in tubular epithelial cells, by the detection of FIV
gag DNA and RNA sequences in nucleic acids extracted by kidney biopsies, and by the presence of scattered interstitial inflammatory and glomerular cells [
7,
14,
31]. Studies in transgenic models suggest that HIV-1 regulatory proteins Vpr and Nef play an important role in HIVAN etiopathogenesis [
2]. Since FIV has no known functional Vpr and Nef homologs the mechanism underlying renal alterations in FIV infection could be different. A possible explanation is that FIV Vif and/or ORF-A genes may replace some functions of HIV-1 Vpr [
2,
44]. Recent studies of HIV have shown that deletions of both Vpr and Vif are required to completely prevent G2 “stalling”, a process in between complete G2 arrest and normal cell-cycle progression [
45,
46]. Similar results were also observed by deleting FIV ORF-A [
44]. These results suggest that FIV Vif and ORF-A may overlap some HIV Vpr and Nef functions and thus contribute, with an unknown mechanism, to the induction of renal alterations.