CD11c+ dendritic cells are a subset of dendritic cells (a type of immune cell) that are only found in human genital tissues, specifically at the epithelial level (the thin layer of tissue that forms the surface) of the vagina, inner foreskin and anus.
One of the lead researchers on this project, Associate Professor Andrew Harman from The Westmead Institute for Medical Research says that the role of these newly discovered CD11c+ dendritic cells is to capture any incoming disease-causing virus or bacteria (pathogen), and then deliver it to CD4 T cells.
“CD4 T cells are responsible for driving an immune response to the pathogen.
Interestingly, they are also the primary HIV target cells in which the virus replicates.
“Once dendritic cells capture a pathogen, they communicate what they have found to CD4 T cells in the lymph nodes, essentially giving the immune system a constant update.
This information prepares the immune system to either tolerate a bacteria or virus, or attack it.
“However, if CD4 T cells fall below critical levels (e.g. in HIV positive patients), then the body is no longer able to mount an immune response, leading to a diagnosis of AIDS.
“Our research team has shown that the newly discovered CD11c+ dendritic cells are more susceptible to HIV infection than any other known dendritic cell.
We have also shown that CD11c+ dendritic cells interact with CD4 T cells more efficiently than any other dendritic cells.
Importantly CD11c+ dendritic cells transfer the virus to CD4 T cells, making them key drivers of HIV infection. As these dendritic cells are so efficient at interacting with CD4 T cells, they are also important vaccine candidates.
The team from The Westmead Institute for Medical Research were able to discover these CD11c+ dendritic cells using donated genital tissues.
Associate Professor Harman says, “We were able to look at the tissue only 30 minutes after it had been surgically removed from the body and also developed ground breaking RNAscope technology which allowed us to watch as living CD11+c dendritic cells took up the virus and delivered it to the CD4 T cells,” says Associate Professor Harman.
According to co-lead author and Executive Director of The Westmead Institute for Medical Research, Professor Tony Cunningham, this discovery has unlocked two new avenues for medical researchers to pursue in the search for more effective HIV treatments.
“This finding opens up a potential for the development of strategies to block the transmission of HIV. If we can block HIV’s ability to bind to the CD11+c dendritic cells, which are often the first immune cells to encounter the HIV virus, then we can stop their ability to transmit the virus to the CD4T cells. In a situation where there are low levels of CD4 T cells, this could stop the virus from spreading.
“Another avenue is to use this new information to develop a HIV vaccine. If HIV fragments or inactivated HIV were targeted at these CD11+c dendritic cells, this would have the potential to prime an immune response against HIV as soon as it enters the body,” says Professor Cunningham.
In macaques, SIV first infects CD4+ T cells where it replicates before spreading to other cells in the tissue (1).
However, results in the macaque SIV model might not translate to human HIV transmission.
Immune genes have co-evolved with pathogens; in particular, innate immune genes in non-human primates and humans show signs of divergent evolution.
Moreover, human and non-human primate microbiota in the female genital mucosa, which affect transmission (3), are different.
Importantly SIV infection of primate immune cells differs from HIV-1.
Although the efficiency of reverse transcription is reduced in non-dividing cells by the host factor SAMHD1, the Vpx accessory protein in SIV and HIV-2 overcomes SAMHD1’s activity by promoting SAMHD1 degradation (4–7).
Even amongst HIV strains and human subjects, HIV strain variations, vaginal infection and inflammation, and genetic variations strongly influence sexual transmission.
The immune cell composition of the human female genital mucosa influences susceptibility to HIV-1 transmission.
In a previous study, we optimized a procedure that combined enzyme digestion and mechanical dissociation to retrieve both lymphoid and myeloid cells efficiently from the genital mucosa without removing cell subtype surface markers (8).
A surprising finding was that CD14+ myeloid cells were the most abundant hematopoietic cells in the cervix, comprising about half of CD45+ mononuclear cells.
Myeloid cells were located exclusively in the subepithelial stroma. Most of the CD14+ cells were conventional CD14+CD11c- macrophages, but about a third were CD14+CD11c+ tissue dendritic cells (DCs), most of which were CD103-CD11b+CX3CR1+DC-SIGN+.
Previous studies had reported that myeloid cells represented only ~10% of the hematopoietic cells in the genital mucosa (9, 10), but this apparent discrepancy could have been due to suboptimal recovery from the tissue stroma with routine protocols for tissue dissociation.
Because of their abundance in the genital tract, myeloid cells and specifically DCs, expressing the HIV-binding co-receptors, CX3CR1 (the fractalkine receptor) and DC-SIGN, might be important in HIV transmission.
It is unclear which human cells first bind HIV and whether myeloid cells contribute to transmission.
Thus, although myeloid cells do not efficiently replicate HIV-1 [reviewed in (13)] they could still be one of the first cells to take up the virus.
Early myeloid cell viral capture could play an important role in transmission both by sensing the virus and inducing innate and adaptive immune responses and by transferring the virus to T cells (14, 15).
In one study lamina propria DCs in human intestinal explants transported HIV-1 inoculated onto the apical surface through the mucosa and transmitted it in trans to blood and intestinal lymphocytes (16).
Another study showed that lamina propria DCs, but not macrophages, in the gut can migrate toward R5-tropic virus to sample luminal virions, retain the virus and thereafter transmit the infection to receptive target cells (17).
Furthermore, a study using single cell suspensions of cells from the lower female genital tract showed that DCs were the first cells to capture the virus, but HIV became predominant in T cells at later time points (18).
Another study from the same group demonstrated that vaginal DCs capture transmitted founder HIV and that vaginal DCs, but not macrophages or CD3+ T cells, transport HIV out of the mucosa and could transfer HIV to vaginal and blood T cells (19).
The same group also showed more recently that CD14+CD11c+ DCs derived from the human genital tract are one of the first immune cells to encounter HIV when a cell suspension of digested tissue is incubated with GFP-labeled HIV-viral-like particles (20) and that ovarian CD14+ cells could be infected with HIV (21).
To examine the cells that first capture HIV within intact female genital tissue, an important site of HIV heterosexual transmission, in this study we looked at infection in explants of human cervical mucosa that preserve the local tissue environment.
We infected healthy donor human cervical tissue explants with JRCSF, a CCR5-tropic clinical isolate of HIV-1, to ask which cells are initially infected.
In particular we wanted to know whether HIV-1, like SIV, first infects CD4+ T cells and amplifies in them.
In some experiments, we compared infection of JRCSF packaged with Vpx (Vpx-JRCSF) with wild-type (WT) JRCSF to examine the role in mucosal infection of the HIV restriction factor SAMHD1, whose degradation is orchestrated by Vpx (6, 7).
To capture the first infected cells, we sorted subpopulations of genital immune cells 20 h after infection and used sensitive qRT-PCR to look at which cell populations contain HIV RNA.
HIV RNA was detected in CD14+ myeloid cells more often than in CD4+ T cells, suggesting that myeloid cells take up HIV early in transmission.
Higher levels of HIV RNA were measured in samples infected with Vpx-JRCSF than with wild-type virus. HIV RNA was present predominantly in CD14+CD11c+ dendritic cells, rather than in CD11c-CD14+ macrophages.
Imaging flow cytometry (IFC) confirmed preferential HIV uptake in cervical myeloid cells 1 day after infection.
Myeloid cells took up the virus, but did not produce new viral particles, because viral RNA levels in CD14+ cells were not reduced by an RT inhibitor, as they were in CD4+ T cells.
We also could not detect any signal for integrated provirus in myeloid cells by nested Alu-gag PCR, further suggesting that captured virus in myeloid cells did not replicate at this early timepoint.
However, CD14+ myeloid cells from HIV-infected cervical explants transmitted the virus to activated blood CD4+ T cells, confirming that the virus taken up by the myeloid cells is infectious.
Thus, resident tissue myeloid cells in the human genital tract mucosa may be the first cells to take up HIV and likely play a role in mucosal transmission.
More information: Kirstie M. Bertram et al, Identification of HIV transmitting CD11c+ human epidermal dendritic cells, Nature Communications (2019). DOI: 10.1038/s41467-019-10697-w
Journal information: Nature Communications
Provided by Westmead Institute for Medical Research