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Quantitative analysis of mononuclear cells expressing human immunodeficiency virus type 1 RNA in esophageal mucosa.

The mucosa of the gastrointestinal tract is presumably an important reservoir for human immunodeficiency virus type 1 (HIV-1), but the level of virus-expressing cells within the mucosa of infected patients is not known. To study this issue, we identified HIV-1 mRNA-expressing (positive) mononuclear cells by in situ hybridization in specimens of esophageal mucosa from eight patients with acquired immune deficiency syndrome (AIDS) and esophageal infections. Such cells were not found in four patients with AIDS and no esophageal disease. Immunocytochemical staining revealed that the mononuclear cells expressing HIV-1 mRNA were lamina propria macrophages. The prevalence of positive cells was measured by triplicate determinations in each of three experiments using an inverse sampling technique. No significant differences in prevalence were found among patients or among experiments. The overall prevalence of HIV-1 mRNA-expressing cells in the esophageal lamina propria was 0.059 +/- 0.01%. This prevalence of cells expressing HIV-1 mRNA in the mucosa of patients with mucosal infections may reflect the local abundance of stimuli such as bacterial endotoxin and certain cytokines capable of inducing viral transcription.

T he mucosa of the gastrointestinal tract plays a critical role in the immunopathogenesis of HIV-1 infection (1,2). Among homosexual men, still the largest population of persons at risk for HIV-1 infection in developed countries, the mucosa offers access to host tissues for both virus-infected cells and cell-free virus. As the largest lymphoid organ in the body, the mucosal lamina propria may also serve as a potential reservoir for HIV-l-infected mononuclear cells. Such cells likely contribute, either directly or indirectly, to the reduction in lamina propria helper/inducer (CD4 +) cells (3), which parallels the reduction in circulating CD4 + T cells. As local immunosuppression progresses, the esophagus and intestinal tract become sites of opportunistic infection(s) (4,5).
Despite involvement of the mucosa in the immunopathogenesis of AIDS, information on the presence of HIV-1 in the gastrointestinal tract mucosa is limited. For example, the level of HIV-1 mRNA-expressing cells in the mucosa has not been determined, yet such cells likely have an important impact on mucosal immune function. Therefore, we used in situ hybridization and immunolabeling to identify HIV-1 KNA-expressing macrophages in the mucosa and then determined the prevalence of these cells in patients with AIDS.

Materials and Methods
MucosalSpecimens. Esophageal biopsy specimens were obtained from eight patients with AIDS who were part of an ongoing study 1541 of gastrointestinal opportunistic infection(s) in AIDS (4,5). The histologic diagnoses for the eight patients were esophagitis due to Candida albicans in six patients, cytomegalovirus in one patient, and C. albicans plus herpes simplex virus in one patient. Esophageal mucosa from four patients with AIDS but no esophageal disease served as control tissue.
In Situ Hybridization. Biopsy specimens were evaluated for HIV-1 RNA by in situ hybridization according to our previously described protocol (6,7). Briefly, esophageal biopsy specimens were fixed in 1.3 M formaldehyde (8), embedded in paraffin, sectioned, and then bonded to silanized microscope slides (3% triethoxy-3-aminopropyl silane in anhydrous acetone followed by washing in distilled water), deparafl~nized, and hydrated in graded alcohols. Probes were synthesized in the antisense and sense configuration, each subclone having two promoters, and then sheared by alkaline hydrolysis to yield 200 base RNA fragments. Probe fragments were combined in equal amounts to make a single antisense and a single sense probe with the sensitivities of "~30-300 copies of target RNA per cell. The sensitivity was determined by experiments in which a known number of cell-free viral particles fixed in a fibrin clot and hybridized with the above pooled probes was shown to yield 2.4-3.0 silver grains per viral genome (Fox, C.H., unpublished observations). Four serial sections from each tissue specimen were probed for HIV-1 RNA with the antisense probe.
The evaluation of each tissue specimen included the following controls: (a) sections of known HIV-l-infected and uninfected brain tissue from another patient; (b) HIV-l-infected and uninfected H9 ceils; (c) a "nonsense" control probe; and (d) two serial tissue sections from each specimen run with the sense probe. Slides were examined by dark-field and bright-field microscopy. Ceils (excluding eosinophfls and cells at the section edge) were considered positive when more than 20 white grains (dark-fild) or black grains (brightfield) were detected overlying a cell in an area of 200 ~tm z in a typical stellate pattern. This procedure was replicated three times in each of three experiments (nine separate determinations for each of eight patients). In the fist experiment, bright-field illumination was used to identify positive cells, and the procedure was unblinded, whereas the second and third experiments were blinded, dark-field illumination was used, and the number of positive cells in the first field with at least one positive cell was counted.
Immunohistochemical Staining. Immunohistochemical staining was performed to identify cells that expressed HIV-1 mRNA. Briefly, formaldehyde-fixed sections of mucosal tissue were stained for I h with the following monoclonal antibodies: OPD4 (Dako, Carpinteria, CA), which reacts with helper/inducer T lymphocytes; HAM56 (Dako), which labds fixed tissue macrophages; or CD21 (Dako), which reacts with mature B cells and follicular dendritic cells. After washing, the slides were treated sequentially for 30 rain with biotinyhted rabbit anti-mlgG and alkaline phosphataselabeled streptavidin (Dako) and then reacted for 10 rain with new fuchsin chromogen substrate (Dako). For double-labeling experiments, slides were first stained by the immunohistochemical procedure and then hybridized with RNA probes to detect HIV-1 mRNA according to the in situ hybridization procedure described above (9,10).
Statistical Analysis. In each slide, nonoverlapping microscopic fields were examined until a field containing one or more positive ceils was found (11)  A square root transformation (14) was applied to the observed values of N + 1/(M + 1) and a three-way analysis of variance carried out (factors: patient, experiment, replicate order within experiment). Normality of the residuals was checked with a Shapiro-Wilk test (15). No significant main effects were found; however, there was a highly significant patient-experiment interaction due primarily to data collected in the second experiment.

Detection of HIV-l-infected Cells in Esophageal Mucosa.
Cells expressing HIV-1 rnRNA were identified by in situ hybridization in biopsy specimens of esophageal mucosa from each of eight patients with AIDS and an opportunistic esophageal infection. HIV-1 mRNA-expressing ceils occurred as single cells or in groups of up to four positive ceils per field (Fig. 1 A). Positive ceils were detected only in the lamina propria (Fig. 1 B) and appeared to be mononuclear leukocytes based on size, morphology, and nuclear configuration. In contrast, HIV-1 mRNA-expressing cells were not detected in the control mucosa from AIDS patients without esophageal disease.   There was, nonetheless, evidence that positive cells tended to form small aggregates since, compared with the corresponding numbers of fields expected with a random distribution of cells (see Table 1, Expected, geometric distribution), there was a defidt of fidds free of such cells (15 vs. 29.6) but an excess of fields with two to four positive cells (31 vs. 19.9). Table 3 shows the average number of calls enumerated per quadrant (i~) for each patient and the corresponding average number of fields counted until a field having at least one positive cell was found (1~). Individual patient prevalences were calculated by introducing the estimates i~ and/q' into the formula at the foot of Table 3. The overall prevalence of mononuclear cells expressing HIV-1 mRNA in esophageal mucosa of patients with AIDS was estimated by taking the arithmetric mean of the individual patient prevalences: mean P+ = 5.86 x 10 -4. The corresponding standard error was 1.02 x 10 -4. A 95% confidence interval for mean P+ is (3.45 -8.26) x 10 -4.

Discussion
Using in situ hybridization, we identified mononuclear cells expressing HIV-1 mRNA in the lamina propria of esophageal mucosa from eight AIDS patients with esophageal opportunistic infections. Double-labeling experiments showed that the productively infected cells were macrophages. The depletion of mucosal CD4 + lymphocytes, which parallels the depletion of circulating CD4 + lymphocytes in patients with AIDS (3), may have contributed to our inability to detect HIV-1 mRNA expressing T lymphocytes in the lamina propria. Quantitative analysis of the hybridized slides revealed that on the average, one HIV-l-infected cell per "~2,000 lamina propria mononuclear cells (0.059%) actively expressed viral RNA at levels detectable by in situ hybridization. We recognize, however, that this percentage may actually underestimate the prevalence of mucosal cells infected with HIV-1, since our technique detected cells expressing viral RNA and not cells containing only integrated or extrachromosomal proviral DNA, which ranges between 0.1% and 13.6% of mononudear cells in blood (16,17).
Although HIV-1 has been identified in an esophageal ulcer (18) and in intestinal mucosa (4,6,7,(19)(20)(21)(22)(23), this is the first quantitative assay of the viral burden in mucosal tissue from patients with AIDS. Moreover, we believe that our evaluation is to date the most reliable estimate of the prevalence of cells expressing HIV-1 transcripts in a body tissue or fluid. The number of negative fields counted during inverse sampling of the hybridized tissue sections was shown to follow a Poisson distribution, and an analysis of variance showed no significant differences in counts of negative fields among the eight patients or three experiments. This method of estimating the prevalence of mRNA positive cells obviates counting the total number of cells in each field. Instead, the average total number is estimated independently, thereby saving appreciable time and effort.
The 0.059% prewalence of HIV-1 mRNA-expressing cells in esophageal mucosa reported here is 6-60 times higher than the apparent frequency (0.001-0.01%) of cells expressing viral RNA in lymph nodes from patients with AIDS (24). In the latter study, however, Harper et al. (24) used a different hybridization protocol and did not quantitate viral burden (number of infected cells) by the inverse sampling technique described above. Ideally, the same in situ hybridization protocol, probes, and quantitative analysis should be used to compare frequencies of HW-1 mRNA-expressing cells in different organs.
Recently, we used protease-digested tissue in hybridization experiments to show that lymph nodes are important reservoirs of free virus during early HIV-1 infection and that this reservoir disappears as the germinal centers involute with advancing disease (9). More recently, Embretson et al. (25) extended these observations by combining in situ hybridization and polymerase chain reaction to show that high numbers of CD4 + cells in the germinal centers and follicular mantle of lymph nodes from patients with early HIV-1 infection were latently infected with HIV-1 DNA. In contrast to these studies, we determined the frequency of productivdy infected (HIV-1 mRNA-expressing) cells among all mucosal cells and performed these studies on tissues from patients with late HIV-1 infection (AIDS and opportunistic infections). Thus, during late HIV-1 disease the expression of HIV-1 by mononudear cells in gastrointestinal mucosa may be different from that of lymph nodes where productively infected cells are no longer present (9,10,25).
The frequency of mucosal cells productively infected with HIV-1 reported here may reflect the local availability of extracellular stimuli capable of activating the cellular transcription factors that regulate viral replication. Stimuli that may be present in the gastrointestinal tract mucosa that are capable of activating lymphocytes and macrophages for HIV-1 expression include bacterial lipopolysaccharide (26,27), although this is still controversial (28,29), and tumor necrosis factor ot (30,31), which is abundantly expressed by mucosal macrophages in some patients (7). Elucidating the local events that promote HIV-1 expression by mononuclear cells in the mucosa will be critical to understanding the role of the gastrointestinal tract as a reservoir for HIV-1.
We thank Drs. Jim Richter, Doug Horst, and Mike Karasik for providing the control tissue specimens.