The kidneys are generally enlarged, even at end stage (Fig. 11.1). In adults, the mean combined kidney weight has been documented to be as high as 500 g (58). In children, the mean weight is usually 1.20 to 1.30 times normal (22). The capsular surface is smooth. The cortex is enlarged and is more pale than the medulla. The parenchyma is swollen. Small cysts, 0.5 to 1.0 mm, are occasionally observed in the cortex or at the corticomedullary junction; they correspond to massively dilated tubules. As a rule, unless hypertension or chronic renal disease existed before the onset of HIVAN, grossly discerned scarring is not a feature.

There are prominent coexisting changes of glomeruli and tubules, often with an important interstitial component. The lesions of glomeruli are most typically, but not always, of the collapsing variant of focal segmental glomerulosclerosis (59). Because HIVAN has both clinical and pathologic manifestations of tubulointerstitial as well as glomerular involvement, the designation “nephropathy” rather than focal and segmental glomerulosclerosis is appropriate. As we view the glomerular abnormalities, they represent predominantly the early lesions of focal segmental glomerulosclerosis (“shift to the left” in the morphogenesis) (29,38,60). Initially, there are hypertrophy and hyperplasia of visceral epithelial cells; these alterations may affect only a few cells of a single lobule (Fig. 11.2A) or virtually all epithelial cells in a glomerulus simulating a crescent (Fig. 11.2B). Parietal epithelial cells have
been shown to contribute to the glomerular epithelial cell hyperplasia in collapsing glomerulopathy (61). Mitotic figures are usually present in some epithelial cells. The abnormal cells may be coarsely vacuolated and characteristically contain many cytoplasmic protein reabsorption droplets (Fig. 11.2C). Capillary walls in affected segments may be wrinkled or collapsed and lumina narrowed. There is relative dilation of the urinary space with segmental or more extensive involvement. As the lesion evolves, there is further narrowing or obliteration of lumina, resulting in solidification of the tufts (Fig. 11.3). In some glomeruli, typical segments of sclerosis develop, with lumina containing foam cells, insudative lesions, or both (Fig. 11.4). In advanced lesions, the visceral epithelial cells remain numerous and enlarged with protein resorption droplets and cytoplasmic vacuoles; the cells are typically arranged as a crown over the solidified tuft or a portion thereof (Fig. 11.5) (5,9,22,29,30,38,58).

There are constant and varied abnormalities of the tubules. Cells display degenerative features, with diminished or absent brush border staining of proximal tubular cells, flattening of cells in many segments of the nephron, and sloughing of cells into the lumina (Fig. 11.6). These changes may be focal or, more commonly, widespread (29,30,38). Lumina of tubules, in all segments of the nephron, are characteristically filled with a precipitate of plasma protein (Fig. 11.7), staining pale or negative with periodic acid-Schiff (PAS) and fuchsin positive with Masson trichrome. Tubular lumina are dilated, some only modestly and others massively, forming microcysts that may be visible on gross examination. Some tubules are so markedly dilated that they dwarf adjacent glomeruli (Fig. 11.8). Bowman spaces, also dilated and in direct continuity with dilated proximal tubules, contain the same precipitate (Fig. 11.9). The luminal precipitates, whether in greatly or slightly dilated tubules, have a scalloped periphery at the interface with apical portions of epithelial cells. Some of the luminal precipitates may contain more densely staining casts, crystals, or desquamated cells or cellular debris. In addition, some nondilated tubules contain typical hyaline casts composed of Tamm-Horsfall protein; as expected, these are strongly PAS positive. Other changes of tubular epithelium include the accumulation of numerous protein reabsorption droplets in proximal cells (Fig. 11.10) (29,38).

The interstitium is edematous and contains a variable infiltrate of lymphocytes, with fewer plasma cells and monocytes (29,30,38). Interstitial fibrosis and tubular atrophy are present as the disease progresses; alternatively, these features may be related to other chronic changes (e.g., hypertension and nephrosclerosis) antedating the development of HIVAN. There are no consistent accompanying alterations of arteries, arterioles, or veins. Hypertension as an integral feature of HIVAN is unusual. Vascular changes of nephrosclerosis are most commonly associated with preexisting hypertension.

The immunofluorescence findings in glomeruli are as would be expected for focal and segmental glomerulosclerosis, including the collapsing variant. In glomeruli with segmental sclerosis, immunoglobulin M (IgM), C3, and C1q are typically found in capillary walls in a coarsely granular to amorphous pattern with a segmental distribution, corresponding in location to the abnormal segments (27,28,36). “Noninvolved” glomeruli usually show negative results on immunofluorescence, although IgM and C3 have been documented in mesangial regions in a granular pattern and with a diffuse distribution in some reports. Most commonly, albumin, but also IgA, IgG, or both, is present in visceral epithelial cells as protein resorption droplets (29,30,38) (Fig. 11.11). Similarly, albumin, with lesser amounts of IgG, IgA, complement, and both light chains, is found in proximal tubular cells as protein resorption droplets. In our study of the composition of the precipitates (“casts”) in dilated tubular lumina, we documented the presence of all plasma proteins sought (IgG, IgA, IgM, C1q, C3, albumin, both light chains) but not Tamm-Horsfall protein

(Fig. 11.12), strongly suggesting their origin to be of the glomerular filtrate. In contrast, typical tubular hyaline casts are composed of Tamm-Horsfall protein, IgA, variable IgM, and both light chains, findings typical for these casts in other settings (62).

Cell marker studies have elucidated the composition of the leukocytic interstitial infiltrate. In the study by D’Agati et al. (30), there were both CD4 and CD8 cells, with a markedly reduced CD4/CD8 ratio (0.23 to 0.77, mean 0.35), similar to findings in the peripheral blood. In contrast, Rey et al. (63), studying mostly patients with full-blown AIDS, identified the infiltrating cells to be predominantly T cells, with both CD4 and CD8 cells present in a ratio of approximately 1.0, considerably different from the situation with circulating blood, which had markedly diminished numbers of CD4 cells and more numerous CD8 cells. These authors documented expression of major histocompatibility complex class II molecules (HLA-DR) on these cells as well as on parenchymal cells (endothelial, glomerular mesangial, and visceral epithelial cells). T cells were rarely identified in tubules or glomeruli. B cells and macrophages represented a minor component of the infiltrate. This study indicates that while AIDS may be associated with considerable CD4 lymphopenia, some organs may harbor a disproportionate number of these cells. Bodi et al. (64) documented that approximately 50% of interstitial leukocytes were macrophages, approximately 3% were B cells, and the remainder were T cells; it is not clear from their report what manifestations of HIV infection the patient population exhibited, making comparison with the other two studies difficult. It is of interest to note that Rey et al. (60,63) found the infiltrate in HIVAN to differ from that of idiopathic focal segmental glomerulosclerosis, which is characterized by considerably higher CD4/CD8 ratios.

The ultrastructural findings reflect two distinctive processes and features: the basic electron microscopic features of the glomeruli and tubulointerstitium in focal segmental glomerulosclerosis/collapsing glomerulopathy (65,66,67) and the fine structural abnormalities of HIV infection (28,30,38,45,62,68,69). Each is discussed separately.

In the case of glomerulopathy, the abnormalities are dominated by pronounced changes in visceral epithelium and mirror very well the features of light microscopy (27,28,36). In affected glomeruli or segments thereof, these cells are often massively enlarged and contain one or more dense, round, secondary lysosomes (Fig. 11.13A); in addition, large and irregular single membrane-bound and seemingly empty vacuoles are present (see Fig. 11.13B). The vacuoles are often responsible for bizarre and irregular shapes assumed by these cells. The vacuoles may be exceedingly large, larger than the caliber of the underlying capillary lumen. The foot processes are usually completely effaced. The epithelial cell cytoplasm is not infrequently partially or completely detached from the basement membrane (Fig. 11.14A); at a later stage, after repair, the cell is separated from the original basement membrane by thin layers of presumably new basement membrane material (Fig. 11.14B).

Basement membranes display varying degrees of wrinkling with luminal narrowing (29,38). Endothelial cells may be swollen. With advancing lesions, capillary lumina may be completely obliterated by basement membrane material. Less commonly, monocytes with considerable cytoplasmic lipid (foam cells) fill the lumina, often in association with degeneration and detachment of endothelial cells. With further progression, plasma protein insudates, in the form of extracellular electron-dense masses, often with incorporated lipid and debris, occlude the lumina (38). Electron-dense deposits are usually absent, although small deposits in the mesangium are present in glomeruli with mesangial IgM and C3 deposits.

There are several ultrastructural abnormalities associated with HIV infection; it should be recognized that they are not limited to the kidneys but are widespread in many organs (68,69,70). Thus, their presence does not define the kidney changes as being indicative of HIVAN but as occurring in an HIV-infected patient. That is, there is a prevalent misconception that when these ultrastructural abnormalities are present in the kidneys, they are indicative only of HIVAN or a lesion
directly related to HIV infection. However, they are indicative only of HIV infection and are present in the kidneys with other lesions, ranging from IgA nephropathy perhaps integrally related to HIV to, for example, diabetic nephropathy in an HIV-infected patient.

The most common abnormality is the tubuloreticular structure (Fig. 11.15A), consisting of reticular aggregates of branching tubules, approximately 25 nm in diameter, within cisternae of the endoplasmic reticulum or the nuclear envelope (68,69). These cytoplasmic structures, also known as tubuloreticular inclusions or interferon footprints, are primarily found in endothelial cells of glomeruli and peritubular capillaries and, to a lesser degree, in arteries, arterioles, and veins; in addition, they may be present in lymphocytes and monocytes (Fig. 11.15B), whether in the interstitium or in vessel lumina. They are typically large and numerous. Tubuloreticular structures are considered to be induced by cellular exposure to interferon-alpha. They are also an integral feature of cells in patients with systemic lupus erythematosus and in patients who are treated with interferon-alpha and may be found in patients with other viral infections (71). In addition, tubuloreticular structures may be identified, although in small numbers, in other forms of renal disease. Therefore, these structures are not specific to HIV infection.

Another ultrastructurally defined lesion, cylindrical confronting cisternae, is emphasized as an important and regularly occurring abnormality (28,69), but it is infrequently noted in renal tissue. These cisternae are long cylinders of fused membranous lamellae also known as test tube- and ring-shaped forms (Fig. 11.16); they almost always occur in cells with tubuloreticular structures, mainly in lymphocytes and monocytes. Confronting cylindrical cisternae are not unique to HIV infection; they also have been documented in multiple sclerosis, systemic lupus erythematosus, and in association with interferon-alpha therapy. Nuclear alterations are not uncommon. Perhaps, the most typical are nuclear bodies; these are nuclear inclusions, with diverse morphologic characteristics, consisting of dense or pale granular or fibrillary aggregates (Fig. 11.17). They are usually divided into five different types based upon structure (68) and are found most often in tubular and interstitial cells, including leukocytes and fibroblasts, where they are frequently quite numerous in a single nucleus.
The type III variant is most commonly present (30,38). Nuclear bodies are not specific to HIV infection; they had been described many years before the onset of the AIDS epidemic. Their composition and function are unknown; they may occur in association with viral infections. At the present time, this group of structures no longer generates the same interest as earlier.

More pronounced nuclear changes also may be present, including granular transformation of nuclei and, less commonly, granulofibrillar transformation. In the former case, the nucleus is completely or partially replaced by a coarsely granular material often associated with disruption of a portion of or the complete nuclear membrane (Fig. 11.18). Concomitant alterations of cytoplasm and organelles, especially mitochondrial swelling, are not uncommon. It should be noted that this change is not an artifact of inadequate fixation, for nuclei and cytoplasm of adjacent cells are not similarly abnormal. This alteration typically affects tubular and interstitial cells (28,36); it is more frequently found in interstitial cells in our experience (38).

Another more dramatic and perhaps related lesion is granulofibrillar transformation of nuclei; the chromatin is replaced by material with a coarse or fine granularity, with interspersed bundles of pale-staining filamentous (“fibrillar”) structures
(Fig. 11.19). While granular transformation is readily seen in biopsy material, it should be noted that granulofibrillar transformation seems to be restricted to postmortem specimens (38,68,69). The nature of the transforming material is not known in either instance. The specificity of granular transformation to HIV infection is not known; we have noted it occasionally in renal biopsies from patients with systemic lupus erythematosus, among other disorders. While cytomegalovirus (CMV) and other viruses have been identified in renal tissue in HIVAN and represent coexisting disease rather than having pathogenetic importance in this disorder, the HIV agent has not yet been described in any renal cell.

The tubular casts of plasma protein are of medium density, with a finely granular to homogeneous appearance, without fibrils typical of Tamm-Horsfall protein (62), and without cytoplasmic or other debris. The casts look the same, regardless of the nephron segment in which they are located (29). Morphologic tubular cell manifestations reflecting acute tubular necrosis and tubular reabsorption of protein are similar to those seen in other nephropathies. The interstitial changes, likewise, are not different from those found in other disease contexts.

The light microscopic features, especially of the collapsing form of focal segmental glomerulosclerosis with tubular cell changes, tubular luminal protein precipitates, and cysti-cally dilated tubules—in conjunction with the ultrastructural findings (particularly, but not exclusively, numerous, large, and widespread tubuloreticular structures)—are a fairly unique combination of findings. Observed together, these are highly suggestive of HIVAN, and HIV infection should be confirmed by testing for HIV with polymerase chain reaction (PCR) or for HIV antibodies serologically. The differential diagnosis includes patients treated with interferon, which can induce a picture of collapsing glomerulopathy with tubuloreticular structures (72). This constellation of findings allows the renal pathologist to suggest a diagnosis of HIV infection in a patient not previously known to harbor the virus (27,28,29,44,45,46). It should be emphasized that the diagnostic criteria must include electron microscopic evaluation as collapsing glomerulopathy clearly occurs in non-HIV-infected persons, some of whom may have other immunodeficiency conditions; if light microscopy alone is used, diagnostic specificity is lost. Thus, incompletely studied renal specimens are likely to provide misleading information.

In the HAART era, tubuloreticular structures are much less commonly found (21,73). Some specimens may be devoid of them, and fewer tubuloreticular structures are present in those biopsies that harbor them. Although the consequence of
these features is that they have a reduced role in pathologic diagnostic specificity, the pathologist is very likely to be aware that the patient is HIV infected at the time of the renal biopsy. Additionally, since the introduction of HAART, the prevalence of the typical morphologic changes of HIVAN has decreased, while the “classical” (non-collapsing) forms of focal segmental glomerulosclerosis have increased likely due to effectiveness of HAART in abrogating the collapsing glomerulopathy of HIVAN (74,75). Over the last several years, the noncollapsing morphology is the leading cause of glomerular disease in HIV-infected individuals followed in one large institution in Paris (26).