Posted by: Maryam Ehteshami and Kasra Hassani
It has been documented that HIV infection can render leishmaniasis harsher and reduce the chances of treatment response. On the other hand, Leishmania infection also accelerates HIV infection and disease progression. In this blog post, we summarize a recent article published in PLoS Pathogens, that explores the mechanism through which Leishmania can help HIV replication. As it turns out, human macrophages are a key part of the equation.
It is no secret that both HIV and Leishmania can infect macrophages. So when Mock et al. wanted to examine the relationship between these two microorganisms, macrophages were the first place they looked.
Macrophages are non-dividing cells with a low nucleotide pool. Nucleotide synthesis is regulated at the S phase, in other words, cell activation. So in resting macrophages the nucleotide levels are very low. Additionally, It was previously thought that human monocytes do not further proliferate once they leave the bone marrow. Recent studies however, have shown that monocytes may be far more heterogeneous than previously thought, and that a subset of them can go on to enter the cell cycle in response to certain stimulations. For example, cells stimulated with GM-CSF (Granulocyte-monocyte colony-stimulating factor) were shown to go on to proliferate.
Interestingly, Mock et al. showed that Leishmania infection can promote monocyte viability and proliferation similar to GM-CSF. It has long been seen that intracellular protozoan parasites such as Leishmania, Trypanosoma and Toxoplasma can inhibit apoptosis of their host cell and thereby increase their lifespan (Heussler et al. 2001). Mock et al. take this to the next step showing that Leishmania might further induce proliferation of the infected monocytes. This could help spreading of the parasitic infection, and indeed Mock et al. show that the cells remain infected after proliferation (Figure 1, PKH dye shows that monocytes are infected with Leishmania). However, if this is good or bad for Leishmania, still needs to be determined, especially that an activated macrophage could be able to kill internalized parasites.
Another theory as to why Leishmania promotes cell cycle progression in macrophages is the following: Leishmania parasites lack the machinery necessary for synthesizing purine nucleotides. Resting cells have low levels of nucleotides. Therefore, by activating the cell cycle, Leishmania ensures that sufficient levels of nucleotides become available for its replication. In fact Mock et al. examined the effect of macrophage infection by Leishmania on expression levels of ribonucleotide reductase (RNR) enzyme. This enzyme is responsible for converting ribonucleotides to nucleotides. Western blotting revealed that RNR levels increased in the presence of Leishmania infection, similar to those elevated levels seen in the presence of GM-CSF. This elevation also corresponded with nucleotide level increases. Overall, this presents one theory for why Leishmania would want to induce macrophage stimulation.
As mentioned earlier, macrophages are also a target for HIV infection. But HIV replicates at a very high rate. And high rate of replication requires high levels of intracellular nucleotides. In fact, the Kim group have previously shown that HIV reverse transcription is severely reduced under conditions which mimic macrophage intracellular levels of nucleotides and that HIV replication is lower in macrophages as compared to T cells (Kennedy et al. 2010) . Based on their observations with Leishmania and HIV, they showed that HIV replication in macrophages may increase when the cells are co-infected with the parasite (Figure 1, Green GFP-HIV proliferation only occurs in GM-CSF-treated or Leishmania-infected monocytes). They also showed that this is the result of Leishmania-induced cell proliferation and increased nucleotide levels (figure not shown). In other words, Leishmania manipulates the macrophage to create a friendlier environment for its own survival and HIV ceases this opportunity and uses these changes in the macrophage for its own gain.
It is likely that there are many different mechanisms involved in Leishmania/HIV co-infection that were not discussed here. Almost certainly many of them involve immune modulation. Here, Mock et al. have shed light on a unique biochemical mechanism for the observed increased infection by either microorganism. As suggested by the authors, it would be interesting to examine other macrophage-infecting microorganisms such as Mycobacterium tuberculosis in this context.
This topic raises many exciting questions. Most fundamentally, what is the underlying mechanism for induction of proliferation in Leishmania-infected monocytes? And what is the implication of monocyte proliferation to spread or control of Leishmania infection? But also, what happens to HIV in macrophages in the absence of a parasitic infection? These could perhaps be the topics of next research projects and next blog posts.
Mock DJ, Hollenbaugh JA, Daddacha W, Overstreet MG, Lazarski CA, Fowell DJ, & Kim B (2012). Leishmania induces survival, proliferation and elevated cellular dNTP levels in human monocytes promoting acceleration of HIV co-infection. PLoS pathogens, 8 (4) PMID: 22496656
Kennedy EM, Gavegnano C, Nguyen L, Slater R, Lucas A, Fromentin E, Schinazi RF, & Kim B (2010). Ribonucleoside triphosphates as substrate of human immunodeficiency virus type 1 reverse transcriptase in human macrophages. The Journal of biological chemistry, 285 (50), 39380-91 PMID: 20924117
Heussler VT, Küenzi P, & Rottenberg S (2001). Inhibition of apoptosis by intracellular protozoan parasites. International journal for parasitology, 31 (11), 1166-76 PMID: 11563357