Precocious thymic involution manifest by epithelial injury in the acquired immune deficiency syndrome

Differences in pathogenicity among strains of the same or different avian leukosis virus subgroups

An efficient induction of wasting disease in chickens by avian leukosis virus (ALV), particularly ALV subgroup C, requires >102 infectious units virus inoculated in mid embryogenesis. The most conspicuous symptoms of the disease were induced by ALV subgroup C; however, significant differences in the occurrence of wasting disease were found among individual members of this subgroup. Almost comparable pathogenicity was exhibited by ALV subgroup D, whereas viruses of subgroups B and A proved to be moderately and almost non-pathogenic, respectively. Using antibodies to cellular antigens, tissue alterations were shown clearly in ALV-C-infected chickens. An essential feature was depletion of lymphocytes in the thymus, bursa and spleen. While the number of dendritic cells in the bursa was increased, their representation in the thymus and spleen was reduced. In the spleen, however, the reduction of dendritic cells concerned only an ellipsoid compartment, which in itself was also markedly reduced. An increased number of macrophages in the thymus and spleen corresponded with the observed general activation of the monocyte-macrophage system. In the spleen, CD4+ T cells were reduced while CD8+ T cells were increased. In agreement with this finding was a failure of chickens to respond to Brucella antigen and an inability of their splenocytes to respond to Concanavalin A, both of which pointed to the damage of immune reactivity. Variation in the pathogenicity among individual ALV strains provides ground for depicting gene sequences playing an important role in ALV acute pathogenicity.

HIV-1 replication and pathogenesis in the human thymus

How HIV replicates and causes destruction of the thymus, and how to restore thymic function, are among the most important questions of HIV-1 pathogenesis and therapy in adult as well as pediatric patients. The thymus appears to function, albeit at reduced levels, throughout the life of adults, to respond to T cell depletion induced by HIV and to be suppressed by HIV. In this review, we summarize recent findings concerning HIV replication and pathogenesis in the human thymus, focusing on mechanistic insights gleaned from studies in the SCID-hu Thy/Liv mouse and human fetal-thymus organ culture (HF-TOC) models. First, we discuss HIV viral determinants and host factors involved in the replication of HIV in the thymus. Second, we consider evidence that both viral factors and host factors contribute to HIV-induced thymocyte depletion. We thus propose that multiple mechanisms, including depletion and suppression of progenitor cells, paracrine and direct lytic depletion of thymocytes, and altered thymocyte selection are involved in HIV-induced pathology in the thymus. With the SCID-hu Thy/Liv mouse and HF-TOC models, it will be important in the coming years to further clarify the virological, cell biological, and immunological mechanisms of HIV replication and pathogenesis in human thymus, and to correlate their significance in HIV disease progression.

Activation of the signal transducer and activator of transcription 1 signaling pathway in thymocytes from HIV-1-infected human thymus

OBJECTIVE

To identify HIV-induced host factors in the severe combined immunodeficient (SCID)-hu Thy/Liv mouse that may contribute to HIV pathogenesis in the thymus.

DESIGN

To identify genes specifically altered by HIV-1 infection using the cDNA microarray assay, SCID-hu Thy/Liv organs derived from the same donors were used. Therefore, no genetic variations existed between HIV and mock-infected samples. In addition, the 12-14 day post-infection timepoint was chosen because no significant thymocyte depletion was detected in HIV-infected Thy/Liv organs, so mRNA from the same cell types could be compared.

METHODS

Using SCID-hu Thy/Liv mice constructed from the same donor tissues, we analysed the expression of 9183 host genes in response to HIV infection with cDNA microarrays. Expression of selected genes with more than threefold induction was confirmed by measuring RNA (reverse transcriptase-polymerase chain reaction; RT-PCR) and proteins.

RESULTS

HIV-1 (JD or NL4-3) infection of the SCID-hu Thy/Liv mouse led to more than threefold induction of 19 genes, 12 of which were IFN-inducible and six were unknown EST clones. We confirmed induction by RT-PCR and protein blots. Both signal transducer and activator of transcription (STAT)1 and STAT2 proteins were induced, and STAT1 was also activated by phosphorylation at the Tyr701 and Ser727 sites in human thymus infected with HIV-JD or NL4-3. Treatment of human fetal thymus organ culture or human thymocytes with recombinant HIV-1 gp120 proteins also led to induction or activation of STAT1.

CONCLUSION

HIV-1 infection of the thymus led to activation of the STAT1 signaling pathway in thymocytes, which may contribute to HIV-1 pathogenesis in the thymus.

Antiviral therapy reduces viral burden but does not prevent thymic involution in young cats infected with feline immunodeficiency virus

The thymus is a major target organ in human immunodeficiency virus type 1 (HIV-1)-infected children and feline immunodeficiency virus (FIV)-infected young cats (G. A. Dean and N. C. Pedersen, J. Virol. 72:9436-9440, 1998; J. L. Heeney, Immunol. Today 16:515-520, 1995; S. M. Schnittman et al., Proc. Natl. Acad. Sci. USA 87:7727-7731, 1990; T. A. Seemayer et al., Hum. Pathol. 15:469-474, 1984; H.-J. Shuurn et al., Am. J. Pathol. 134:1329-1338, 1989; J. C. Woo et al., J. Virol. 71:8632-8641, 1997; J. C. Woo et al., AIDS Res. Hum. Retrovir. 15:1377-1388, 1999). It is likely that the accelerated disease process in children and cats is due to infection of the thymus during the time when generation of naive T lymphocytes is needed for development of the mature immune system. Zidovudine (ZDV) monotherapy, which is used to prevent and treat perinatal HIV-1 infection (R. Sperling, Infect. Dis. Obstet. Gynecol. 6:197-203, 1998), previously had been shown to reduce viral burden in FIV-infected young cats (K. A. Hayes et al., J. Acquir. Immune Defic. Syndr. 6:127-134, 1993). The purpose of this study was to evaluate the effect of drug-induced reduction of viral burden in the thymus on virus-mediated thymic involution and peripheral blood CD4 decline using FIV-infected cats as a model for pediatric HIV-1 infection. Eight-week-old cats were randomly assigned to uninfected, saline-treated; uninfected, ZDV-treated; FIV-infected, saline-treated; and FIV-infected, ZDV-treated groups. Parameters measured included blood lymphocyte numbers, viral load in blood and thymic tissue, and thymic histopathology. While the viral burden was significantly reduced by ZDV monotherapy in peripheral blood lymphocytes, plasma, and thymus, thymic lesions were similar for the treated and untreated FIV-infected cats. Further, markedly lowering the viral burden did not increase blood CD4 lymphocyte numbers or prevent their decline. The data suggest that an inflammatory process continued in spite of reduced virus replication. These observations imply that reducing virus load and limiting thymic inflammation are separate factors that must be addressed when considering therapeutic strategies aimed at preserving thymic function.

Pathogenesis of primary R5 human immunodeficiency virus type 1 clones in SCID-hu mice

We studied the replication and cytopathicity in SCID-hu mice of R5 human immunodeficiency virus type 1 (HIV-1) biological clones from early and late stages of infection of three patients who never developed MT-2 cell syncytium-inducing (SI; R5X4 or X4) viruses. Several of the late-stage non-MT-2 cell syncytium-inducing (NSI; R5) viruses from these patients depleted human CD4(+) thymocytes from SCID-hu mice. Earlier clones from the same patients did not deplete CD4(+) thymocytes from SCID-hu mice as well as later clones. We studied three R5 HIV-1 clones from patient ACH142 in greater detail. Two of these clones were obtained prior to the onset of AIDS; the third was obtained following the AIDS diagnosis. In GHOST cell infection assays, all three ACH142 R5 HIV-1 clones could infect GHOST cells expressing CCR5 but not GHOST cells expressing any of nine other HIV coreceptors tested. Furthermore, these patient clones efficiently infected stimulated peripheral blood mononuclear cells from a normal donor but not those from a homozygous CCR5Delta32 individual. Statistical analyses of data obtained from infection of SCID-hu mice with patient ACH142 R5 clones revealed that only the AIDS-associated clone significantly depleted CD4(+) thymocytes from SCID-hu mice. This clone also replicated to higher levels in SCID-hu mice than the two earlier clones, and a significant correlation between viral replication and CD4(+) thymocyte depletion was observed. Our results indicate that an intrinsic property of AIDS-associated R5 patient clones causes their increased replication and cytopathic effects in SCID-hu mice and likely contributes to the development of AIDS in patients who harbor only R5 quasispecies of HIV-1.

Inhibition of thymopoiesis of CD34+ cell maturation by HIV-1 in an in vitro CD34+ cell and thymic epithelial organ culture model

The mechanisms by which HIV-1 affects thymopoiesis were determined by preincubating CD34+ cells or cultured thymic epithelial (CTE) cells with lymphotropic (T-) and monotropic (M-) strains of HIV-1 in an in vitro CTE organ and CD34+ cell coculture model that allows for analysis of development of thymocytes and mature T cells. When purified CD34+ cells were precultured with either T- or M-tropic strains of HIV-1, thymopoiesis was impaired in a two-week coculture manifested by decreased cell number of thymocytes generated. However, the percentages of thymocyte subpopulations were comparable to control uninfected cocultures. Furthermore, HIV infection of thymocytes was predominantly observed in the CD44+CD3- population. However, in a four-week coculture experiment, HIV infection and depletion of more mature thymocytes were also observed. When CTE cells were preincubated with T- and M-tropic strains of HIV before addition of CD34+ cells, the number of thymocytes and subpopulations of thymocytes at early and later stages of maturation were markedly decreased. Furthermore, CD34+ and CD44+CD3- cells become HIV-infected. In summary, HIV-1 infection inhibited thymocyte maturation at early stages of thymocyte maturation CD44+CD25-CD3-. In addition, HIV also depleted later stages of CD4+ thymocyte subpopulations.

Induction of MHC Class I Expression on Immature Thymocytes in HIV-1-Infected SCID-hu Thy/Liv Mice: Evidence of Indirect Mechanisms

The SCID-hu Thy/Liv mouse and human fetal thymic organ culture (HF-TOC) models have been used to explore the pathophysiologic mechanisms of HIV-1 infection in the thymus. We report here that HIV-1 infection of the SCID-hu Thy/Liv mouse leads to the induction of MHC class I (MHCI) expression on CD4+CD8+ (DP) thymocytes, which normally express low levels of MHCI. Induction of MHCI on DP thymocytes in HIV-1-infected Thy/Liv organs precedes their depletion and correlates with the pathogenic activity of the HIV-1 isolates. Both MHCI protein and mRNA are induced in thymocytes from HIV-1-infected Thy/Liv organs, indicating induction of MHCI gene expression. Indirect mechanisms are involved, because only a fraction (<10%) of the DP thymocytes were directly infected by HIV-1, although the majority of DP thymocytes are induced to express high levels of MHCI. We further demonstrate that IL-10 is induced in HIV-1-infected thymus organs. Similar HIV-1-mediated induction of MHCI expression was observed in HF-TOC assays. Exogenous IL-10 in HF-TOC induces MHCI expression on DP thymocytes. Therefore, HIV-1 infection of the thymus organ leads to induction of MHCI expression on immature thymocytes via indirect mechanisms involving IL-10. Overexpression of MHCI on DP thymocytes can interfere with thymocyte maturation and may contribute to HIV-1-induced thymocyte depletion.

Population biology of HIV-1 infection: viral and CD4+ T cell demographics and dynamics in lymphatic tissues

Human immunodeficiency virus-1 (HIV-1) is usually transmitted through sexual contact and in the very early stages of infection establishes a persistent infection in lymphatic tissues (LT). Virus is produced and stored at this site in a dynamic process that slowly depletes the immune system of CD4+ T cells, setting the stage for AIDS. In this review, I describe the changes in viral and CD4+ T cell populations in LT over the course of infection and after treatment. I present recent evidence that productively infected CD4+ T cells play an important role in establishing persistent infection from the onset, and that the LT are the major reservoir where virus is produced and stored on follicular dendritic cells (FDCs). I discuss the methods used to define the size of viral and CD4+ T cell populations in LT and the nature of virus-host cell interactions in vivo. These experimental approaches have identified populations of latently and chronically infected cells in which virus can elude host defenses, perpetuate infection, and escape eradication by highly active antiretroviral treatment (HAART). I discuss the dramatic impact of HAART on suppressing virus production, reducing the pool of stored virus, and restoring CD4+ T cell populations. I discuss the contributions of thymopoiesis and other renewal mechanisms, lymphatic homeostasis and trafficking to these changes in CD4+ T cell populations in LT, and conclude with a model of immune depletion and repopulation based on the limited regenerative capacity of the adult and the uncompensated losses of productively infected cells that treatment stems. The prediction of this model is that immune regeneration will be slow, variable, and partial. It is nonetheless encouraging to know that even in late stages of infection, control of active replication of HIV-1 provides an opportunity for the immune system to recover from the injuries inflicted by infection.

Analysis of the adult thymus in reconstitution of T lymphocytes in HIV-1 infection

A key question in understanding the status of the immune system in HIV-1 infection is whether the adult thymus contributes to reconstitution of peripheral T lymphocytes. We analyzed the thymus in adult patients who died of HIV-1 infection. In addition, we studied the clinical course of HIV-1 infection in three patients thymectomized for myasthenia gravis and determined the effect of antiretroviral therapy on CD4(+) T cells. We found that five of seven patients had thymus tissue at autopsy and that all thymuses identified had inflammatory infiltrates surrounding lymphodepleted thymic epithelium. Two of seven patients also had areas of thymopoiesis; one of these patients had peripheral blood CD4(+) T-cell levels of <50/mm3 for 51 months prior to death. Of three thymectomized patients, one rapidly progressed to AIDS, one progressed to AIDS over seven years (normal progressor), whereas the third remains asymptomatic at least seven years after seroconversion. Both latter patients had rises in peripheral blood CD4(+) T cells after antiretroviral therapy. Most patients who died of complications of HIV-1 infection did not have functional thymus tissue, and when present, thymopoiesis did not prevent prolonged lymphopenia. Thymectomy before HIV-1 infection did not preclude either peripheral CD4(+) T-cell rises or clinical responses after antiretroviral therapy.

The human thymus. A chimeric organ comprised of central and peripheral lymphoid components

The human thymus is a lymphoepithelial organ in which T cells develop during fetal life. After maturation and selection in the fetal thymic microenvironment, T cells emigrate to peripheral lymphoid tissues such as the spleen, gut, and lymph nodes, and establish the peripheral T cell repertoire. Although the thymus has enormous regenerative capacity during fetal development, the regenerative capacity of the human postnatal thymus decreases over time. With the advent of intensive chemotherapy regimens for a variety of cancer syndromes, and the discovery that infection with the Human Immunodeficiency Virus (HIV) leads to severe loss of CD4+ T cells, has come the need to understand the role of the human thymus in reconstitution of the immune system in adults. During a recent study of the thymus in HIV infection, we observed many CD8+ T cells in AIDS thymuses that had markers consistent with those of mature effector cytotoxic T cells usually found in peripheral immune tissues, and noted these CD8+ effector T cells were predominantly located in a thymic zone termed the thymic perivascular space. This article reviews our own work on the thymus in HIV-1 infection, and discusses the work of others that, taken together, suggest that the thymus contains peripheral immune cell components not only in the setting of HIV infection, but also in myasthenia gravis, as well as throughout normal life during the process of thymus involution. Thus, the human thymus can be thought of as a chimeric organ comprised of both central and peripheral lymphoid tissues. These observations have led us to postulate that the thymic epithelial atrophy and decrease in thymopoiesis that occurs in myasthenia gravis, HIV-1 infection, and thymic involution may in part derive from cytokines or other factors produced by peripheral immune cells within the thymic perivascular space.

The human thymus. A chimeric organ comprised of central and peripheral lymphoid components

The human thymus is a lymphoepithelial organ in which T cells develop during fetal life. After maturation and selection in the fetal thymic microenvironment, T cells emigrate to peripheral lymphoid tissues such as the spleen, gut, and lymph nodes, and establish the peripheral T cell repertoire. Although the thymus has enormous regenerative capacity during fetal development, the regenerative capacity of the human postnatal thymus decreases over time. With the advent of intensive chemotherapy regimens for a variety of cancer syndromes, and the discovery that infection with the Human Immunodeficiency Virus (HIV) leads to severe loss of CD4+ T cells, has come the need to understand the role of the human thymus in reconstitution of the immune system in adults. During a recent study of the thymus in HIV infection, we observed many CD8+ T cells in AIDS thymuses that had markers consistent with those of mature effector cytotoxic T cells usually found in peripheral immune tissues, and noted these CD8+ effector T cells were predominately located in a thymic zone termed the thymic perivascular space. This article reviews our own work on the thymus in HIV-1 infection, and discusses the work of others that, taken together, suggest that the thymus contains peripheral immune cell components not only in the setting of HIV infection, but also in myasthenia gravis, as well as throughout normal life during the process of thymus involution. Thus, the human thymus can be thought of as a chimeric organ comprised of both central and peripheral lymphoid tissues. These observations have led us to postulate that the thymic epithelial atrophy and decrease in thymopoiesis that occurs in myasthenia gravis, HIV-1 infection, and thymic involution may in part derive from cytokines or other factors produced by peripheral immune cells within the thymic perivascular space.

Pig MHC Mediates Positive Selection of Mouse CD4+ T Cells with a Mouse MHC-Restricted TCR in Pig Thymus Grafts

Remarkably normal immune function and specific T cell tolerance to discordant xenogeneic donors can be achieved by grafting fetal pig thymus and liver (FP THY/LIV) tissue to T cell and NK cell-depleted, thymectomized (ATX) mice. To determine whether or not host class II MHC molecules participate in the positive selection of mouse CD4+ T cells in FP THY/LIV grafts, we compared their development in ATX “AND” TCR-transgenic mice with positive selecting or nonselecting host MHC genotypes. Mouse TCR-transgenic CD4 single positive T cells repopulated the periphery significantly and to a similar extent in both T/NK cell-depleted, ATX AND mice with positive-selecting or nonselecting MHC backgrounds after grafting with FP THY/LIV. Therefore, MHC molecules from a widely disparate xenogeneic species can positively select T cells bearing a host class II MHC-restricted TCR without a contribution from the host MHC. These results, in combination with previous studies performed in this model, suggest that the T cell repertoire that is generated by the combination of positive selection on xenogeneic MHC and negative selection on both recipient and xenogeneic porcine MHC is tolerant of both donor and recipient and has sufficient cross-reactivity with host MHC/foreign peptide complexes to confer a high level of immunocompetence. The results have implications for the potential clinical applicability of xenogeneic thymic transplantation and also suggest a predominant role for the TCR recognition of species-conserved MHC residues in positive selection.

High prevalence of thymic tissue in adults with human immunodeficiency virus-1 infection

The thymus in adults infected with the HIV-1 is generally thought to be inactive, both because of age-related involution and viral destruction. We have revisited the question of thymic function in adults, using chest-computed tomography (CT) to measure thymic tissue in HIV-1-seropositive (n = 99) or HIV-1-seronegative (n = 32) subjects, and correlating these results with the level of circulating CD4(+) and CD8(+) T cells that are phenotypically described as naive thymic emigrants. Abundant thymic tissue was detectable in many (47/99) HIV-1-seropositive adults, aged 20-59. Independent of age, radiographic demonstration of thymic tissue was significantly associated with both a higher CD4(+) T cell count (P = 0.02) and a higher percentage and absolute number of circulating naive (CD45RA+CD62L+) CD4(+) T cells (P < 0.04). The prevalence of an abundant thymus was especially high in younger HIV-1-seropositive adults (</= 39 yr) with CD4 counts in the range 300-500 cells/microl and in older subjects (> 40 yr) regardless of CD4 count (P = 0.03). These studies suggest that the thymus is functional in some but not all adults with HIV-1 disease.

Transgenic mice expressing human immunodeficiency virus type 1 in immune cells develop a severe AIDS-like disease

We have constructed transgenic (Tg) mice expressing the entire human immunodeficiency virus type 1 (HIV-1) coding sequences in cells targeted by HIV-1 infection in humans. These Tg mice developed a severe AIDS-like disease leading to early death (< 1 month). They developed muscle wasting, severe atrophy and fibrosis of lymphoid organs, tubulointerstitial nephritis, and lymphoid interstitial pneumonitis. In addition the expression of RANTES was increased in various tissues of these Tg mice relative to that in the normal controls. Disease appearance was correlated with the levels of transgene expression. The numerous pathologies observed in these mice are remarkably similar to those observed in human AIDS and, more specifically, in pediatric AIDS.

HIV-1 pathogenesis and therapeutic intervention in the SCID-hu Thy/Liv mouse: a model for primary HIV-1 infection in the human thymus

The SCID-hu Thy/Liv mouse is a model for the analysis of human thymopoiesis. It has been constructed by engrafting fragments of human fetal liver and thymus into the immunodeficient C.B-17 scid/scid (SCID) mouse. The resulting 'Thy/Liv' organ promotes long-term differentiation of human T cells. Given the apparently normal physiology of the SCID-hu Thy/Liv organ, it has been used to explore the pathophysiologic mechanisms of HIV-1 infection in vivo, and to test therapeutic modalities such as anti-HIV-1 drugs and haematopoietic stem cell (HSC)-based gene therapy. In this review, I will summarise what we have learned from the SCID-hu Thy/Liv model, with a focus on recent findings in HIV-1 replication and therapy. Unique HIV-1 determinants have been identified which are required for replication in the Thy/Liv organ but not for replication in PBMC or in T cell lines in vitro. The mechanism of HIV-1 induced thymus depletion is not clear. It is correlated with high levels of HIV-1 replication. Both direct and indirect mechanisms may be involved. In addition to preclinical evaluation of anti- HIV-1 drugs, the SCID-hu Thy/Liv mouse has also been successfully used to test the feasibility of HSC-based gene therapy. A number of improved SCID-hu models have been constructed to meet different requirements. Using these SCID-hu Thy/Liv models, current/future efforts will provide insightful information for understanding pathogenesis and designing therapeutic interventions against HIV-1 infection in humans, especially in paediatric patients. Copyright 1997 John Wiley & Sons, Ltd.

Simple i.v. inoculation of HIV-1 to Thy/Liv SCID-hu mice induce reproducible HIV infection with narrowing of medulla in human thymic implant

Human fetal thymus/liver engrafted SCID mice were constructed and studied for its susceptibility to HIVBRU infection by i.v. inoculation which seemed to represent an appropriate route of HIV infection in vivo. By the i.v. inoculation of HIV, the medulla in the engrafted thymus narrowed significantly when compared with that of the human thymic implant from virus-uninoculated mice. Further, immunohistochemical staining indicated the presence of HIV antigen predominantly in thymic epithelial cells in medulla of the engrafted thymus. Polymerase chain reaction (PCR) assays resulted in amplifications of HIV genome in the implanted grafts as well as in lymph nodes and PBMC. The virus infections to the implants were confirmed biologically by coculturing with PHA-stimulated human PBMC and the graft cells from the HIV-inoculated SCID-hu mice. Thus, the i.v. inoculation of HIV into Thy/Liv SCID-hu mice induce narrowing of medulla of the engrafted thymus and may become an efficient and useful tool for screening candidate anti-HIV agents.

Development of a method of thymocyte differentiation of bone marrow-enriched CD34+CD38- cells in postnatal allogeneic cultured thymic epithelia to evaluate immunodeficiency disorders

An in vitro model of CD34+CD38- stem cell (SC) differentiation in postnatal cultured thymic epithelia fragment (CTEF) cocultures is described. Sequential phenotypic analysis of the progeny of the SC-CTEF demonstrated predominantly thymocytes and minor populations of promyelocytes, monocytes and natural killer cells. Triple-positive CD3+CD4+CD8+, double-positive CD4+CD8+, and mature single-positive CD4+ and CD8+ T cells, which were TCR alpha beta+, were identified indicating normal thymocyte maturation. In kinetic studies, mature single-positive CD4+ T cells increased from 29% of total cells at one week to 54% at four weeks of coculture. These findings demonstrate that coculture of bone marrow-derived SC and allogeneic cultured thymic epithelia in vitro results in continuous normal predominantly thymocyte differentiation. The SC-CTEF cocultures were then infected with two different strains of human immunodeficiency virus. CD4+ thymocytes were markedly decreased. However, inhibition of early thymocyte maturation steps was also suggested by the presence of increased triple-negative and CD44+CD25-CD3-thymocytes and decreased CD44+CD25+ thymocytes. This model system of thymocyte maturation will be useful in the evaluation of primary T cell immunodeficiency disorders, gene therapy of SC and pharmacological augmentation of thymic function.

Arrest of in vitro T cell differentiation of normal bone marrow-derived CD34+ stem cells with thymic epithelial fragments from children with AIDS

A novel approach is presented to assess the ability of thymic tissues obtained from children with end stage AIDS to attract normal bone marrow (BM)-derived CD34+ (lineage negative) stem cells (SCs) and support lymphopoiesis in vitro. Chemokinesis of BM-derived CD34+ SCs was analyzed by time-lapse videomicroscopy to ascertain whether an alteration in SC motility could contribute to abnormal thymopoiesis under conditions of HIV infection. The migration of SCs derived from an HIV+ donor into thymic tissue was not significantly altered compared to normal controls, as were normal SCs migrating toward thymic epithelial cell monolayers derived from an HIV+ patient. Thymic tissue obtained from children with AIDS contained nests of CD34+ SCs identified by immunofluorescence, indicating SC homing to the thymus is apparently supported in HIV infection. The ability of HIV-affected thymic epithelial fragments to support lymphopoiesis was determined by examining the initial thymocyte populations present, compared to thymocytes produced de novo in T cell-depleted thymic fragments, following a single pulse of lineage negative CD34+ CD38- SCs. In comparison to normal controls, thymocytes derived from the HIV-affected thymic epithelial fragment coculture had an increased percentage of triple negative thymocytes (28% of lymphocytes from HIV-affected tissue versus 1.5% in controls, p < 0.01) and a decreased percentage of double and single positive CD4+ thymocytes. However, CD3+CD8+ TCR alpha beta + expression was comparable to control cultured thymic epithelial fragments indicating that HIV-affected thymic epithelia were capable of supporting the development of the CD8+ lineage. In an effort to extend the information obtained to date from the histological examination of HIV-affected thymic tissue, select patient thymic tissues were maintained in culture to evaluate the capacity of undifferentiated thymic epithelial cell guirlandes to differentiate in vitro. A partial regeneration of certain subpopulations of the thymic epithelium defined by TE-4 monoclonal antibody (mAb) and CDR2 mAbs occurred during the in vitro culture. The epithelial and mesenchymal components of thymic tissues were distinguished by immunostaining for keratins (indicative of epithelium) and vimentin (a mesenchymal marker). Further evaluation of the modulation of HIV thymus, with respect to the testing of new therapeutic strategies on SCs, will be possible with this in vitro model.

T cell differentiation/maturation of CD34+ stem cells from HIV-seropositive hemophiliacs in cultured thymic epithelial fragments

The clinical manifestations of AIDS are predominantly due to the cellular and humoral immune dysfunction caused by HIV infection, and thymic dysplasia caused by HIV infection probably contributes to the T cell lymphopenia. In the present study, T cell differentiation and/or maturation was assessed when enriched CD34+ stem cells (SCs or SC) purified from bone marrow of HIV-seropositive hemophiliacs were cocultured with allogeneic cultured thymic epithelial fragments (CTEFs). When HIV-seropositive hemophiliacs' enriched CD34+ SC were cocultured with allogeneic CTEFs, acquisition of the T cell phenotypic markers CD7, CD2, CD3, CD4, CD8 and T cell receptor for antigen (TCR) alpha beta was observed from cells harvested from the culture media peaking at approximately 28 days. Origin of the differentiated and matured T cells from the CD34+ SC was confirmed by labeling the SC with 5-(and -6)-(((4-chloromethyl)benzoyl)amino)tetra-methyl-rhodamine (CMTMR), a fluorescent cytoplasmic dye, and detecting fluorescence in the differentiated and matured T cell by flow cytometry. In one experiment, CMTMR labeling was omitted and double positive CD4+CD8+ and triple positive CD3+CD4+CD8+ thymocytes were identified. These studies confirmed that thymocyte differentiation/maturation from SC had occurred. In addition, T cells obtained from the CD34+ SC and CTEF cocultures proliferated to phytohemagglutinin stimulation maximally with stem cell donor antigen-presenting cells (APCs) and also proliferated to pooled B cells in a mixed lymphocyte culture (MLC). Furthermore, the T cells produced were tolerant to thymus donor B cell HLA antigens (p < 0.025); though there was slight MLC reactivity to autologous stem cell donor B cell HLA compared to thymic B cells (p < 0.025). These T cells demonstrated positive self-alloreactivity to stem cell HLA antigens in four of nine persons, though decreased compared to pool B cell alloantigens. Furthermore, in three experiments, responsiveness to stem cell donor B cells subsequently disappeared upon further duration of CD34+ SC-CTEF coculture. These studies suggested that CD34+ SC gave rise to accessory cells populating the thymus that contributed to HLA restriction. To further evaluate this hypothesis, two different donors of CD34+ SC were cultured simultaneously with thymic epithelial fragments and MLC reactivity was then examined toward APC of the stem cell donors. In these experiments, T cells responded to stimulation with HLA antigens of the pool B cells and did not respond to thymus donor B cells. In six of eight experiments, the chimeric SC-CTEF T cells did not respond to stimulation with B cells of either stem cell donor. These studies suggest that HLA restriction and tolerance were induced by cells of the stem cell donor as well as the thymic epithelial cell HLA antigens. In summary, these studies demonstrated that HIV-infected hemophiliac bone marrow-derived nonadherent CD34+ SC were capable of differentiating and/or maturing into T cells when cocultured in a normal allogeneic thymic environment. Furthermore, the T cells derived from derived CD34+ SC were capable of differentiating into T cells when cocultured in a normal allogeneic thymic environment, proliferated maximally with APCs from the stem cell donor and were tolerant of thymic HLA class II antigens, and to a lesser degree to stem cell donor B cell HLA antigens.

HIV-1-induced thymocyte depletion is associated with indirect cytopathogenicity and infection of progenitor cells in vivo

Direct and indirect cytopathic mechanisms have been proposed to account for the loss of CD4+ T cells after infection with human immunodeficiency virus type 1 (HIV-1). We report here that HIV-1 infection of the human thymus in vivo results in thymocyte depletion by at least two different mechanisms. Thymocytes within multiple stages of differentiation are induced to die of apoptosis; most of these cells are uninfected. Additionally, thymopoiesis is interrupted by direct infection and destruction of intrathymic CD3-CD4+CD8- progenitor cells. These mechanisms are differentially induced by distinct isolates of HIV-1.

HIV-1 infection of the thymus: evidence for a cytopathic and thymotropic viral variant in vivo

As thymocyte infection may represent one of the mechanisms responsible for CD4+ T lymphocyte depletion in HIV-1-infected individuals, we studied the occurrence of HIV-1 infection in the thymus in vivo. Thymus (THYPD) and peripheral blood (PBLPD) primary viral isolates were obtained from an HIV-1-infected patient; restriction pattern analysis revealed the presence of a viral variant (THY) in the thymus isolate, from which biological viral clones containing this variant were obtained by limiting dilution infection of Molt-3 cells. The biological phenotype of the viral isolates and THY clones was studied in different cell lines and primary cultures. PBLPD, THYPD, and THY clones could efficiently infect T cell lines; the thymic variant showed a higher cytopathic activity in T cell lines, and a higher replication capacity in both unfractionated and CD4+CD8(+)-enriched primary thymocytes. Sequence analysis of the viral population patterns in vivo confirmed the presence of the THY variant in the thymic compartment, and revealed that the degree of V3 loop heterogeneity was higher in the thymocytes of the patient than in the peripheral blood lymphocytes. In addition to confirming thymocyte infection in vivo, our data also indicate that a differential distribution of viral variants may occur among different body compartments in a single individual; the emergence of cytopathic and tissue-specific variants in the thymus may play a relevant role in the pathogenesis of HIV-1 disease.

Viral DNA burden and decline in percentage of CD4-positive cells in the lymphoid compartment of SIV-infected macaques

The decline in CD4+ cells and increased viral DNA and RNA burden in the blood of human immunodeficiency virus (HIV)-infected individuals have been used as closely related correlates of disease progression. However, little is known about levels of total or unintegrated viral DNA in lymphoid tissue of HIV-infected patients and how they relate to CD4+ cell decline or disease progression. Exploiting the similarities between HIV- and simian immunodeficiency virus (SIV)-induced disease, we examined lymphoid organs and peripheral blood from SIV-infected macaques for total (pol) and unintegrated 2-LTR circular viral DNA by polymerase chain reaction (PCR). Two SIV isolates (SIVmac/251 and SIVmne/E11S) that differ markedly in their biological and clinical properties were studied. The results indicate that total viral DNA burdens vary considerably between isolates. There was no strong association between total viral DNA levels and CD4% in lymphoid tissues when isolates were compared and death was not associated with any particular level of viral pol DNA. In contrast, accumulation of unintegrated viral DNA was closely associated with decline in CD4/CD8 ratios in lymphoid organs and AIDS. The appearance of both pol and unintegrated viral DNA in thymus of infected macaques also emerged as one of the single best correlates or possible predictors of advanced disease yet studied. Their roles in pathogenesis are discussed.

Vitamin E supplementation modulates cytokine production by thymocytes during murine AIDS

Female C57BL/6 mice were infected with LP-BM5 retrovirus, causing murine AIDS, which is functionally similar to human AIDS. Retrovirus infection targeted the thymus, producing altered T cell differentiation via the dysregulation of thymocyte cytokine production. Human AIDS causes vitamin deficiencies, therefore the effects of dietary vitamin E supplementation were determined on the kinetics of cytokine production by concanavalin A-stimulated thymocytes in uninfected normal mice and mice with murine AIDS. Dietary supplementation, with a 15-fold increase in vitamin E (160 IU/l) in the liquid diet (National Research Council), modulated interleukin-2 (IL) production in both uninfected mice and retrovirus-infected mice. Vitamin E significantly reduced the level of IL-4 secretion in the uninfected mice at 4 and 8 weeks, but not at 12 and 16 weeks. It also significantly reduced IL-4 production, elevated by retrovirus infection. Vitamin E significantly reduced IL-6, and interferon-gamma production increased in murine AIDS. The effects of dietary vitamin E on concanavalin A-induced proliferation of thymocytes were consistent with the finding of changes in IL-2 secretion. No effects of dietary vitamin E on thymus weight were observed in uninfected or retrovirus-infected mice, whereas vitamin E significantly increased serum and thymic vitamin E concentration, which had been reduced by retrovirus infection. These data indicate that dietary vitamin E supplementation can modulate cytokine production by thymocytes, affecting T cell differentiation, especially during retrovirus-induced immune dysfunction.

Multifactorial nature of human immunodeficiency virus disease: implications for therapy

The immunopathogenic mechanisms underlying human immunodeficiency virus (HIV) disease are extremely complex; the disease process is multifactorial with multiple overlapping phases. Viral burden is substantial and viral replication occurs throughout the entire course of HIV infection. Inappropriate immune activation and elevated secretion of certain cytokines compound the pathogenic process. Profound immunosuppression ultimately occurs together with a disruption of the microenvironment of the immune system, which is probably unable to regenerate spontaneously. Thus, therapeutic strategies in HIV disease must not be unidimensional, but rather must be linked to the complex pathogenic components of the disease and must address where feasible each of the recognized pathogenic processes for the possibility of therapeutic intervention.

Human immunodeficiency virus infection of the human thymus and disruption of the thymic microenvironment in the SCID-hu mouse

Infection with the human immunodeficiency virus (HIV) results in immunosuppression and depletion of circulating CD4+ T cells. Since the thymus is the primary organ in which T cells mature it is of interest to examine the effects of HIV infection in this tissue. HIV infection has been demonstrated in the thymuses of infected individuals and thymocytes have been previously demonstrated to be susceptible to HIV infection both in vivo, using the SCID-hu mouse, and in vitro. The present study sought to determine which subsets of thymocytes were infected in the SCID-hu mouse model and to evaluate HIV-related alterations in the thymic microenvironment. Using two different primary HIV isolates, infection was found in CD4+/CD8+ double positive thymocytes as well as in both the CD4+ and CD8+ single positive subsets of thymocytes. The kinetics of infection and resulting viral burden differed among the three thymocyte subsets and depended on which HIV isolate was used for infection. Thymic epithelial (TE) cells were also shown to endocytose virus and to often contain copious amounts of viral RNA in the cytoplasm by in situ hybridization, although productive infection of these cells could not be definitively shown. Furthermore, degenerating TE cells were observed even without detection of HIV in the degenerating cells. Two striking morphologic patterns of infection were seen, involving either predominantly thymocyte infection and depletion, or TE cell involvement with detectable cytoplasmic viral RNA and/or TE cell toxicity. Thus, a variety of cells in the human thymus is susceptible to HIV infection, and infection with HIV results in a marked disruption of the thymic microenvironment leading to depletion of thymocytes and degeneration of TE cells.

Thymic pathology in primary and secondary immunodeficiencies

For the sake of clarity and in agreement with the World Health Organization immunodeficiency classification, it is important to distinguish the congenital, inherited malformative lesions called generically 'thymic dysplasia' from the secondary, acquired changes, designated under the broad term of 'severe thymic atrophy'. Thymic dysplasia represents the archetype of thymic changes in cellular immunodeficiency, since there is no example of a thymic dysplasia associated with a normal T-cell function. Thymic dysplasia is observed in several inherited diseases, the most frequent of them being severe combined immunodeficiency. More than the depletion of lymphoid cells, the lack of differentiation of the thymic epithelium, responsible for the absence of Hassal's corpuscles, is the main and constant feature of this condition. Thymic dysplasia underscores the crucial role of the thymic epithelium in the normal differentiation of the T-cell population. Severe thymic atrophy is secondary to various causes, including prolonged protein malnutrition and immunosuppressive or cytotoxic drugs, graft versus host reaction and, chiefly today, chronic viral infection, especially with HIV-1. The morphological changes are similar and are characterized by a partial lymphoid depletion, involving mainly the CD1+ population, necrosis and calcification of epithelial cells, the frequent presence of plasma cells and, more significantly, fibrohyaline changes of the basement membrane of the vessels and thymic epithelium. The severity of the atrophic changes and the immunodeficiency-related manifestations depend on the duration of the aetiological factors and, more significantly, with their early occurrence, within the first months of life. The mechanisms underlying thymic atrophy are poorly understood. A primary impairment of lymphoid cells seems at present to be the most likely hypothesis.

Alteration from T- to B-cell tropism reduces thymic atrophy and cytocidal effects in thymocytes but not neurovirulence induced by ts1, a mutant of Moloney murine leukemia virus TB

The ts1 mutant of Moloney murine leukemia virus TB causes degenerative neurologic and immunologic disease in mice, characterized by development of spongiform encephalomyelopathy resulting in hindlimb paralysis, marked thymic atrophy associated with immunodeficiency, and generalized body wasting. To investigate the pathogenesis of the thymic atrophy caused by ts1, we constructed a chimeric virus, ts1-Cas(NS), in which a major portion of the U3 region of the long terminal repeat of ts1, a T-lymphotropic and neurovirulent murine leukemia virus, was replaced by the corresponding U3 region of Cas-Br-E, a B-lymphotropic and neurovirulent murine leukemia virus. In FVB/N mice, ts1-Cas(NS) induced paralytic and wasting disease with incidence, severity, and latency similar to that induced by ts1, but it failed to cause thymic atrophy as severe as that observed in ts1-infected mice. Furthermore, thymocytes cultured from ts1-Cas(NS)-infected mice died at a much slower rate than those of ts1-infected mice. The U3 substitution in ts1-Cas(NS) specifically diminished the ability of the virus to replicate in the thymus, whereas viral replication in the spinal cord was not significantly affected; thus, neurovirulence was not changed. The correlation of reduced thymic atrophy with decreased thymic viral titers and the decreased ability of ts1-Cas(NS) to cause thymocyte death in mice suggest strongly that the marked thymic atrophy in ts1-infected mice is not an indirect effect occurring secondary to neurodegenerative and wasting disease but is a direct cytopathic effect of high-level viral replication in the thymus.

Immunological dysregulation of lymph nodes in AIDS patients

Changes in immune competent tissues of the HIV-1-infected person reflect to a certain extent the kind and intensity of immunological dysregulations. The diagnostic approach, however, must include immunophenotyping of cells, immunovirological studies of virus distribution in diseased tissues, and functional tests in addition to classical morphology. The latter technique alone just serves as a crude screening method since structural lesions in lymphoid tissues do not permit discrimination from other HIV-independent immune deficiency and autoimmune disorders. Although the overall appearance of lymph nodes in HIV infection and in chronic autoimmune disorders, such as collagen vascular diseases (e.g., rheumatoid arthritis and systemic lupus erythematosus), is similar, immunophenotyping shows a progressive loss of CD4 cells in HIV infection yet a quantitative increase in this cell population in autoimmune disorders (Krueger 1985a). In addition, there are other persistent active infections by lymphotropic viruses (e.g., EBV or HHV-6) which can cause structural and cellular changes in lymphoid tissues closely resembling HIV-induced lesions (Krueger et al. 1988b; Krueger 1985b). The pathological diagnosis therefore nedds to be supplemented by serological studies and--in selected cases--by in situ hybridization for the demonstration of viral genome. Southern blotting for viral DNA can only detect high numbers of viral genome copies in tissue extracts, not in which cell population the virus resides (e.g., malignant cells vs associated "normal" cells), while the polymerase chain amplification reaction, the most sensitive of all (Buchbinder et al. 1988), cannot yet differentiate between latent and (disease-related) active infection. Taking into consideration the above-described precautions in the evaluation of lymphatic lesions, there are a number of characteristic changes which reflect well the sequelae of HIV infection itself and of the ensuing immune dysregulation. Progressive loss of CD4 cells in the paracortex of lymph nodes and in the peripheral blood leads to inversion of the CD4/CD8 ratio. Loss of demonstrable CD4 cells is probably the consequence not only of cell lysis by HIV-1 infection (note: discrepancy between HIV-1 genome positive cell numbers and depletion of CD4 cells) but also of decreased CD4 marker synthesis in infected cells (Stevenson et al. 1987). In this context it is interesting that Fouchard et al. (1986) were able to show HIV expression in CD8 cells and theorized that these developed from infected CD4 cells which subsequently lost the CD4 epitope and expressed CD8.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for susceptibility of intrathymic T-cell precursors and their progeny carrying T-cell antigen receptor phenotypes TCR alpha beta + and TCR gamma delta + to human immunodeficiency virus infection: a mechanism for CD4+ (T4) lymphocyte depletion

Individuals infected by the human immunodeficiency virus type 1 (HIV-1) demonstrate progressive depletion and qualitative dysfunction of the helper T4 (CD4+) cell population. Mechanisms proposed for attrition of CD4+ T cells include direct cytopathicity of these mature cells following infection as well as infection of early T-lymphocyte progenitors. The latter mechanism could lead to failure to regenerate mature functioning CD4+ T cells. The present study determines the susceptibility of thymocytes at various stages of maturity to infection with HIV-1. Various normal thymocyte populations were inoculated with HIV-1, including unfractionated (UF), CD3- CD4- CD8- ["triple negative" (TN)], CD4+ CD8+ ["double positive" (DP)] thymocytes, and thymocyte populations obtained by limited dilution cloning. Cultures were studied for the presence of HIV-1 DNA by polymerase chain reaction in addition to examination for reverse transcriptase activity. We determined that transformed T-cell and thymocyte cell lines completely lacking CD4 were not susceptible to infection by HIV-1, whereas all of the following lines were: UF thymocytes (70-90% CD4hi+); DP thymocytes (99% CD4hi+); TN thymocytes (0% CD4hi+); and TCR alpha beta +, TCR gamma delta +, or CD16+ CD3- (natural killer) thymocyte clones expressing variable levels of CD4 and representing the progeny of TN thymocytes. [TCR alpha beta + and TCR gamma delta + refer to the chains of the T-cell antigen receptor (TCR), and CD4hi refers to a strong rightward shift (greater than 30 linear channels) of the CD4 curve on flow cytometric analysis compared with control.] Monoclonal antibodies (mAbs) to CD4 (T4a epitope) but not to CD3 (T3) were capable of blocking infection of mature and immature CD4hi+ thymocytes. Moreover, anti-CD4(T4a) mAbs also inhibited infection of CD4hi- TN thymocytes, indicating that these T-cell precursors--despite their apparent "triple negativity" (CD3- CD4hi- CD8-)--expressed sufficient CD4 molecules to become infected. Cell sorter analysis with a panel of CD4 mAbs demonstrated a mean shift of the mean fluorescence channel (MFC) with CD4 mAbs on TN thymocytes of 6 +/- 4 MFC units. Thus, intrathymic T-cell precursors and their progeny representing many stages of T-cell ontogeny are susceptible to infection by HIV-1, including early TN thymocytes, which express very low levels of CD4. Infection of multiple stages and multiple subsets of the T-cell lineage in man, mediated via the CD4 molecule, may explain the inability of the T-cell pool to regenerate in the setting of progressive HIV infection.

Effects of infection by HIV-1, cytomegalovirus, and human measles virus on cultured human thymic epithelial cells

A tissue culture system for the growth of human fetal and infantile thymic epithelial (TE) cells has been established and characterized. We have investigated the effects of infection of these cells by human cytomegalovirus (CMV), measles virus, and human immunodeficiency virus type-1 (HIV-1). In the case of CMV, morphological changes were apparent by 2-4 days after viral inoculation of infantile TE cells. CMV-related antigens were detected by immunofluorescence after 12 days, and progeny infectious CMV was recovered from culture media after 18 days. Following infection by measles virus, distinctive, multinucleated giant TE cells appeared in both cultures of fetal and infantile TE cells. Measles virus-inoculated TE cells displayed an altered phenotype, as revealed by reaction with monoclonal antibodies with specificity for a variety of TE markers. Finally, infection of TE cells by HIV-1 resulted in cellular disarrangement, increased numbers of Hassall's corpuscles, and multinucleated giant cells. An increase in the number of cells reactive with monoclonal antibodies, specific for Hassall's corpuscles, was observed in the case of cells infected by either measles virus or HIV-1. These findings suggest that a variety of different viruses can successfully infect thymic epithelial tissue. Because of the important role of the thymus in development of the immune system, it is reasonable to conclude that viral infection of thymic tissue might play an important role in virus-mediated suppression of immune responsiveness.

Necropsy findings in rhesus monkeys experimentally infected with cultured simian immunodeficiency virus (SIV)/delta

Lesions induced in rhesus monkeys by different isolates of simian immunodeficiency virus (SIV)/Delta were studied at necropsy. Four groups of monkeys were inoculated with SIV/Delta isolated from other experimentally infected rhesus monkeys, while one group was inoculated with SIV/Delta from an asymptomatic mangabey monkey. Three rhesus isolates and the mangabey isolate were virulent, killing 75-100% of infected monkeys. One rhesus isolate, which had been extensively passaged in vitro, was attenuated but was restored to virulence by single animal passage. Clinically, infected monkeys had lymphadenopathy, splenomegaly, diarrhea, and a rash. Most monkeys died of enteric disease. The following lesions were seen: weight loss, thymic atrophy, lymphoid atrophy, bone marrow hyperplasia, encephalitis, colitis, amyloidosis, hepatitis, glomerulosclerosis, and the presence of syncytial cells. One Rh Epstein-Barr virus (EBV)-related lymphoma occurred. Opportunistic agents were identified: cytomegalovirus, adenovirus, Cryptosporidia, and Pneumocystis. Shigella and Campylobacter often caused colitis.

Histo- and immunopathological features of terminal AIDS. An autopsy case of a Japanese man with neurological signs as initial symptoms

An autopsy case of a 37-year-old Japanese man, confirmed as an AIDS patient infected by an undetermined route of transmission, is presented. The initial symptoms of full-blown AIDS in this case were neurological, and the patient died of severe pneumonia 9 months after onset. The main histo- and immunopathological features were a marked depletion of helper-inducer T cells and dendritic reticulum cells in the lymphoid tissues, opportunistic infections, and some neuropathologic changes. Very few cells, possibly macrophages, immunoreactive with a monoclonal antibody (VAK-5) against HIV-gag protein P24 were found in the mediastinal lymph nodes. Numerous pathogens had induced opportunistic infections in many organs: severe and generalized cytomegalovirus infection, Pneumocystis carinii pneumonia, bronchopneumonia (possibly due to Pseudomonas aeruginosa), candidiasis in the tongue and oral cavity, and atypical mycobacteriosis in the pulmonic hilar lymph nodes. Vascular proliferation was found in the perinodal regions of some lymph nodes, but this was not neoplastic vascular proliferation compatible with that of localized Kaposi's sarcoma.

Autopsy findings in a Japanese patient with acquired immunodeficiency syndrome

A 37-year-old Japanese man was diagnosed as having acquired immunodeficiency syndrome (AIDS) on the basis of a homosexual history, generalized lymphadenopathy, marked decrease of helper/inducer T cells in the peripheral blood, and positive serum antibody (antibodies) against human immunodeficiency virus (human T-lymphotropic virus type III/lymphadenopathy-associated virus). Autopsy showed severe depletion of lymphocytes in the lymphoid organs including the systemic lymph nodes, spleen, and gut-associated lymphoid tissue. Pneumocystis carinii pneumonia and adrenal cytomegalovirus infection ('necrotizing adrenalitis') were identified, but there was no neoplasm. Postmortem findings in AIDS were reviewed in the literature.

Thymic lesions in fatal infectious mononucleosis

Thymic lesions were studied in 35 patients with fatal infectious mononucleosis (FIM) including 21 males with the X-linked lymphoproliferative syndrome (XLP) and 14 non-XLP patients. Six patterns based on the lymphocyte content and status of Hassall's bodies were observed: massive lymphoproliferation effaced the architecture and Hassall's bodies (HB) in 6 cases. Epstein-Barr virus (EBV)-laden B cells surrounded by cytotoxic T cells and natural killer cells were found in one of these cases. Only 5 of the 35 thymuses contained normal-appearing HB. Seven showed a moderate reduction in HB. Rarely, multinucleated giant cells and hyaline globules were seen where one might expect HB to reside. Plasma cells and macrophages were generally abundant. Eight displayed a marked depletion of HB. In 7, no HB were recognized. Massive necrosis was seen in one of these cases. Stress involution was encountered in only two patients, both of whom had sporadic fatal IM. Thymic lesions and alterations were similar but less extensive in sporadic FIM. These morphological studies, taken in context with clinical and experimental reports, suggest that destruction of thymic epithelium may contribute to the progression of immune defects seen in XLP following EBV infection. The destruction of HB that we observed was similar in appearance to lesions in several other immune deficiency disorders.

The functional and histological basis for graft-versus-host-induced immunosuppression

The GvH reaction resulting from the injection of parental strain cells into adult F1 hybrids suppresses both cell-mediated and humoral immune responses and is dependent on the donor-host combination and the number of parental cells used to induce the GvH reaction. The early suppression is due, at least in part, to the increased number of macrophages and the activation of suppressor macrophages which act directly on the T-helper cell and perhaps the B-cell as well. The macrophage suppression is associated with an increase in PGE production. The long-term T-cell immunodeficiency is mediated by GvH-induced thymic dysplasia resulting in a block or an arrest in T-cell differentiation and deficient IL-2 production. The B-cell immunodeficiency is associated with both a decrease in B-cell production from lymphoid progenitors and a decrease in CFU-s production. The GvH reaction induces 2 types of thymic lesion, a stress-related effect causing atrophy of the thymic cortex and a cytolytic process causing severe-to-moderate lesions in the thymic medulla as a consequence of injury to medullary epithelial cells and a loss of Hassall's corpuscles (thymic dysplasia). By employing the NK-cell-deficient beige mutation, it was shown that the severe-to-moderate thymic medullary lesions occurred in F1 mice only in those transplant situations in which the donor inoculum was of the +/bg genotype, regardless of the genotype of the recipient. It is proposed that activation of parental T cells may contribute to the early immunosuppressive events; however, the relatively permanent immunosuppression appears to be associated with NK-like effector cells which are capable of causing injury to lymphoid and epithelial tissue, especially epithelium of the thymic medulla. These studies raise the possibility that the GvH reaction may contribute to some T- and B-cell immunodeficiencies observed in the SCID and AIDS syndromes, as well as in patients following bone marrow transplantation.

Immunopathology of AIDS (acquired immune deficiency syndrome) observations in 75 patients with pre-AIDS and AIDS

Sequential changes in the morphology of immunocompetent and other organs, in the accompanying immunocytological and immunovirological measurements are described in 75 patients with lymphadenopathy syndrome and with AIDS. The data presented demonstrate a stepwise development of the disease from a hyperimmunization syndrome to T-cell immune deficiency. Excessive antigenic stimulation by a large number of infectious organisms or by transfusion of blood and blood products account for antigenic overloading and hyperimmunization. Among such infectious organisms are certain viruses which per se interfere with cells of the immune system as for instance Epstein-Barr virus, cytomegalovirus, and HTLV3. Developing immunological incompetence will favor the persistence of these and other infectious organisms enhancing the damage of the immune reactivity and finally allow lethal infections or malignant tumors to occur.

Acquired immunodeficiency syndrome with severe gastrointestinal manifestations in Haiti

29 patients (19 males and 10 females) in Haiti were diagnosed as having acquired immunodeficiency syndrome. Their clinical presentation was characterised by unexplained chronic diarrhoea, prolonged fever, extreme weight loss, anorexia, and severe infections. The infectious agents included: Candida albicans (27 patients), Mycobacterium tuberculosis (7 patients), Cryptosporidium (11 patients), Pneumocystis carinii (2 patients), cytomegalovirus (4 patients), and herpes virus (3 patients). In 1 woman Kaposi's sarcoma developed during the course of her disease. Immunological studies of 20 patients revealed profound cell-mediated immune deficiency with cutaneous anergy, marked decrease in the number of T helper cells, and impairment of lymphocyte proliferation. 18 patients died.