Circulating monocytes have the capacity to mature into either macrophages or dendritic cells in tissue, both of which play important roles in the induction and effector phase of immune response. In TB, the macrophages are the central player in the host-bacteria interaction as the main mycobacterial reservoir. In HIV disease, monocyte lineage cells are one of the two main cell types (along with CD4+ T-cells) in sustaining intracellular HIV infection. Monocytes are heterogeneous population with three functionally distinct subsets namely classical, intermediate and non-classical monocytes. The three subsets exist in a continuum, and have a certain plasticity or flexibility to develop into multiple roles depending on the local and tissue environment.
In the current study we sought to evaluate the frequencies of these three subsets in participants with TB, HIV and TB/HIV co-infection. While previous studies had shown that the intermediate and non-classical monocyte subsets were elevated relative to classical monocytes, very little had been done in these disease groups regarding more comprehensive characterization of these subsets. In particular, we wished to quantitate the expression of multiple sets of cell surface and intracellular molecules of high relevance using multi-parameter flow cytometry from a functional point of view.
In publication I, we evaluated Toll-like receptors (TLRs) expression in each of the study cohorts. TLRs are vital pattern recognition receptors by monocyte lineage cells and signal the induction of crucial functions. We focused on three such TLRs (TLR2, TLR4 and TLR9) which have been shown to be involved in many monocyte lineage cell interactions with mycobacterial and HIV infections. We observed enhanced expression of TLR2 and TLR4, but not TLR9 in TB and HIV. TLR4 was particularly high in patients with TB, but also in HIV. We observed comparable increase of TLR4 irrespective of monocyte subset. However, TLR2 expression exhibited a different pattern. Levels among the most prominent classical monocyte subsets were identical in all four cohorts, healthy controls (HC), HIV, TB, and TB/HIV co-infection. In contrast, TLR2 expression was significantly elevated in both participants with HIV and TB, but not with participants with TB/HIV co-infection in the intermediate monocyte subset. We also observed correlations between TLRs and plasma cytokines that were disease and TLR specific.
In publication II, we observed elevated chemokine receptors (CRs) expression which above healthy controls and exhibit a pattern of disease preference. Thus, CCR2 and CX3CR1 were the highest in participants with TB, followed by HIV and TB/HIV co-infection, whereas CCR4 and CCR5 were highest in participants with HIV, and less elevated in TB. CCR2 and CX3CR1 are critical for migration of monocytes to sites of TB infection, as determined by murine models. CCR4 and especially CCR5 have been implicated in migration of cells to distant organs but more as co-receptors for HIV infection. Thus, the observed pattern of CRs expression in these monocytes in different disease states would predict greater availability of these cells or their receptors for interaction with either TB or HIV organisms. From the perspective of the pathogen this would lead to enhanced “substrate”, whereas from the perspective of the host, this could lead to greater immune potential. As a final point, we also observed that the pattern of disease association of CRs was independent of the monocyte subset.
In publication III, we explored the expression of Programmed cell death-ligand 1 (PDL1) on the three monocyte subsets. Like many of the other molecules we have addressed in this thesis, PDL1 expression was enhanced in participants with HIV, TB, and TB/HIV co-infection. Among participants with HIV, PDL1 was correlated with HIV-1 viral load. The enhanced expression was apparent in all three subsets, but it was particularly prominent in the intermediate monocyte subset. Moreover, PDL1 expression was the highest in participants with TB/HIV co-infection. The implications behind these observations is that the subset thought to have the greatest potential for T cell antigen presentation had the highest levels of the T cell down-regulatory PDL1 molecule in the cohort of patients particularly participants with TB/HIV co-infection. Participants with TB/HIV co-infection have the greatest potential to be immuno-compromised and as a result the very need for enhanced not depressed APCs function. In addition, we also observed the PDL1 levels were correlated with multiple plasma, mostly pro-inflammatory,cytokines. We analyzed cytokine mRNA levels of total monocytes to address the source of the cytokines but mRNA levels did correlate with neither plasma cytokine nor PDL1 levels.
Considering all the phenotype analysis in each of the three studies together we could see two patterns emerging. In one scenario,surface molecules expression patterns were disease specific but independent of monocyte subset expression. In other words, whatever the underlying mechanism(s) involved in their regulation, those mechanisms apparently acted similarly in all three subsets. In another scenario, expression of surface molecules showed disease specific patterns, but molecules were particularly enhanced in the intermediate monocyte subsets. These findings imply that there exist mechanisms to modulate surface phenotypes and functions that are unique to a given subset.
In conclusion, we have comprehensively defined the density of multiple molecules expressed by different subsets of monocytes and explored their differences in participants with TB, HIV and TB/HIV co-infection, as well as their correlations with microbial indices and plasma cytokines. Many molecules levels were elevated to some extent in all disease cohorts, but we observed patterns of expression which were particularly elevated in TB (CCR2, CX3CR1, and TLR2), those in HIV (CCR4, CCR5) and those on both (TLR4, PDL1). Molecule-disease associations were either independent of monocyte subset, or most readily revealed in a single monocyte subset. TB/HIV co-infection did not follow a consistent pattern in association with monocytes markers, in some cases more resembling TB, in others HIV, in others neither. Finally, to proof one possible mechanism of association between disease and monocyte phenotype, we explored correlations between monocyte markers and plasma cytokines. We observed significant positive and negative associations, frequently unique to a single monocyte subset or disease cohort, such as TB/HIV co-infected cohort. Collectively, the results imply that there are likely multiple mechanisms involved at many levels regulating the phenotype and function of monocytes, and these differ in different disease states.:Abbreviations ............................................................................................................ 3
Abstract ..................................................................................................................... 4
1. Introduction ........................................................................................................... 6
1.1 Epidemiology of Tuberculosis and Human Immunodeficiency virus ................... 6
1.2 The immunological response to TB and HIV ....................................................... 7
1.2.1 Innate immunity of TB ...................................................................................... 7
1.2.2 Innate immunity of HIV .................................................................................... 11
1.2.3 Immune checkpoint regulation in TB and HIV.................................................. 13
1.3 The role of monocytes in TB, HIV and TB/HIV ................................................... 14
1.3.1 Monocytes ....................................................................................................... 14
1.3.2 Abnormalities of monocytes in TB ................................................................... 15
1.3.3 Abnormalities of monocytes in HIV ................................................................. 16
1.3.4 Abnormalities on monocytes in TB/HIV co-infection ....................................... 17
1.4 The rationale for the thesis ................................................................................ 17
2. Objectives ............................................................................................................ 19
3. Publications .......................................................................................................... 20
4. Summary .............................................................................................................. 65
References ............................................................................................................... 69
Annex I: Author contribution ..................................................................................... 76
Annex II: Declaration of independent writing of the work ......................................... 77
Annex III: Curriculum Vitea ....................................................................................... 78
Annex V: Acknowledgment........................................................................................ 82
Annex VI ................................................................................................................... 84
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:91902 |
| Date | 05 June 2024 |
| Creators | Mekasha, Wegene Tamene |
| Contributors | Universität Leipzig |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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