HIV INFECTION OF THYMOCYTE SUBSETS
The Thymus as a Reservoir of HIV
HIV antigens, transcripts, and genomes have been identified in the human thymus by immunohistochemistry, in situ hybridization, and polymerase chain reaction (PCR), respectively.
Futhermore, productive infection of thymocytes was demonstrated by the observation that HIV+ thymic tissue could transfer virus to permissive cell lines.1 In addition, the thymus may serve as a reservoir of latently infected cells. In an animal model of HIV, infected severe combined immunodeficient (SCID)-hu mice bearing human fetal thymus and liver implants demonstrated that HIV infects activated DP thymocytes during selection and becomes latent as the cells enter quiescence.2 These quiescent thymocytes exit to the periphery and produce virus upon activation. In contrast, the macaque thymus is not thought to be a reservoir for simian immunodeficiency virus (SIV).3 These conflicting results can be explained by the fact that SIV co-receptors are not highly expressed in the macaque thymus, whereas HIV co-receptors are found at high levels in the human thymus. HIV infects cells by using the envelope protein gp120, which binds to CD4 and the co-receptors CXCR4 and CCR5 on target cells. M-tropic stains of HIV use CD4 and CCR5, and T-tropic strains use CD4 and CXCR4. Logically, different HIV strains have varying abilities to infect thymocyte subsets according to co-receptor expression. In the SCID-hu model, M-tropic strains of HIV preferentially infect thymic stromal cells, whereas T-tropic strains infect developing thymocytes.4Infection of Thymocytes
Although it is agreed that HIV infects the thymus, controversy exists over which thymocyte HIV (Table 4.1) subsets are directly infected. Some insight has been gained from studies monitoring CD4 and co-receptor expression on thymocytes.5,6 DN thymocytes, by definition, do not express CD4.
They also lack expression of CCR5, but subsets express CXCR4. Although it was suggested that HIV can infect DNs in a CD4-dependent manner,7 these cells may have been ISPs misidentified as DNs. ISP thymocytes that express low levels of CD4 are CXCR4+ and CCR5. Evidence suggests that these ISPs can be infected by HIV.1,8 Notably, DP thymocytes are efficiently infected by HIVTABLE 4.1
Cellular Targets of HIV in the Thymus
| Cell Type | Direct Infection in vitro | Virus Present in vivo |
| Hematopoietic stem cell | /+ | /Low |
| DN | +/? | + |
| ISP | ++ | ++ |
| DP | +++ | +++ |
| SP CD4 | ++ | ++ |
| SP CD8 | /? | + |
| TEC | +/ | |
| Macrophage | ++ | |
| DC | ++ |
Notes: +, yes;, no; +/, conflicting data; ?, inconclusive.
due to the fact that they express CD4 at high levels, are CXCR4+, and express low levels of CCR5.7 SP CD4 thymocytes also express high levels of CD4 but express less CXCR4 and CCR5. HIV can infect SP CD4 thymocytes,1 but it is possible that infection occurs more efficiently at the DP stage and that virus persists throughout maturation. Similarly, SP CD8 thymocytes have been shown to harbor HIV,9-10 but the majority of these cells may have become infected earlier in development.
When infected SP CD4s were cocultured with uninfected SP CD8s, HIV was not transferred to the SP CD8s.11 Because SP CD8 thymocytes lack CD4 and express CXCR4 and CCR5 at extremely low levels, direct infection of these cells by HIV may be inefficient. More comprehensive studies demonstrated that reporter genes inserted into the HIV genome can be detected in DN, DP, SP CD4, and SP CD8 thymocytes in vitro9 as well as in the SCID-hu system.10 In addition to thymocytes of the αβ T cell lineage, developing NK cells, NKT cells, and γ T cells were shown to be susceptible to HIV infection in the SCID-hu model.12Infection of Thymic Stromal Cells
Thymic stromal cells, such as epithelia, macrophages, and dendritic cells (DCs), may also be affected during HIV infection. Thymic macrophages can be directly infected by HIV in vitro, and as a result, they produce excessive levels of cytokines.13 HIV also infects human thymic DCs. After infection, these cells undergo morphological changes associated with apoptosis and produce unidentified soluble factors that are toxic to neighboring thymocytes.14 Another study demonstrated that HIV infects thymic DCs in a CD4-dependent manner and, furthermore, showed that HIV could be transmitted to mature T cells during a mixed lymphocyte reaction.15 The third important component of thymic stroma that is affected during infection is the epithelium, but reports disagree as to whether HIV acts directly or indirectly on these cells. Thymic epithelial monolayers cultured from human tissue can be infected by HIV independent of CXCR4 and CCR5.16 In contrast, independent studies have suggested that thymic epithelial cells (TECs) are not infected directly by HIV but that culture supernatant from infected TECs enhances viral replication in infected T cells.13,17 Upon analysis, the body of evidence reveals the difficulty in determining which thymic subsets are directly infected by HIV during T cell development in vivo and at what stage infection occurs.
Furthermore, it is unclear exactly which cells may be infected by disparate strains of HIV. The strongest evidence suggests that DP, SP CD4, DCs, and macrophages are the primary targets for HIV in the thymus.THYMIC ARCHITECTURE
Thymic Histology
Located in the upper anterior mediastinum, the thymus derives from epithelial outgrowths of the ventral wing of the third pharyngeal pouch. Microscopic analysis reveals that each lobe of this bilobed organ contains numerous smaller lobules. Surrounding the thymus is a collagenous capsule contiguous with capsular septa that extend inwardly into the organ, enveloping the smaller lobules. Blood vessels located along the septa nourish the thymus and provide an entry point for lymphoid progenitors. Upon histologic examination, the thymus contains two distinct regions: the cortex and the medulla. The outer cortex is highly cellular, containing a delicate network of convoluted thymic epithelium and numerous developing thymocytes. The inner medulla contains a densely packed epithelial network interspersed with fewer developing lymphocytes. Also characteristic of the medulla are degenerate epithelial cells forming spiraled structures termed Hassal’s corpuscles. Thymic antigen presenting cells (APCs) such as macrophages and dendritic cells are also interspersed throughout the medulla and cortex. Although the functional intricacies of thymic architecture are not well understood, it is clear that differentiation of immature T cells is highly dependent on complex interactions with cortical and medullary epithelia during various stages of development.
Thymic Pathology during HIV Infection
HIV infection has dramatic effects on thymic histology. Autopsy results from HIV-infected individuals document the presence of B cell follicles in the thymus18 and infiltration of mature B and T cells into perivascular spaces.19 Epithelial cell death is also observed in HIV-infected thymuses, along with disappearance of Hassall’s corpuscles and disruption of the corticomedullary junction.19 Additionally, premature thymic involution has been observed in HIV+ infants with concomitant depletion of DP and SP CD4 thymocytes.20 In the SCID-hu model, HIV infection leads to thymic dysplasia, epithelial disorganization, and depletion of DP thymocytes.21
The Aging Thymus
In many ways, thymic pathology associated with HIV resembles the effects of aging on the thymus.
In general, the thymus actively generates large numbers of T cells from birth until puberty, at which point thymic function declines throughout life. However, thymic involution begins as early as 1 year of age. During this process, the thymic epithelium folds in on itself and degenerates, as adipose tissue infiltrates the interlobular septa. Consequently, the thymus can no longer accommodate the numbers of thymocytes it once did, and T cell output declines. However, thymopoiesis continues at low levels well past the seventh decade of life.22 Given that children exhibit such robust thymic activity, HIV may have more severe effects on the young thymus than the aged thymus. In fact, HIV+ children showing signs of premature thymic atrophy have a poor prognosis and progress rapidly to AIDS.23