Though HIV-1 is the retrovirus that gets all the press these days, it was not the first human retrovirus discovered. The ‘first’ human retrovirus was discovered in 1980— Human T-lymphotropic virus, HTLV.
HTLV has a couple of things in common with HIV-1– Though it is not a lentivirus, it is still ‘complex’, meaning it has gag, pol, env and a couple accessory genes. And it likes to infect activated CD4+ T-cells like HIV-1.
But there is an interesting difference– You cant take a bunch of HTLV viruses, throw them on CD4+ T cells, and infect the cells. That is, ‘free’ virus is relatively non-infectious. To contrast that with HIV-1, I have to be careful to not accidentally put too much virus on my CD4+ T cells, or else Ill come back in 24 hours and all my cells will be infected and dead.
How the hell can a virus not infect the cell it infects? What??
The answer is, of course, mind control.
Remember when I last talked about virus motility?
To use HIV-1 as an example, HIV-1 doesnt just hang out in your vagina waiting for a CD4+ T-cell to wander by (which is good, cause thats not going to happen). Vaginas have sentinal dendritic cells that pick up HIV-1 viruses (like a friggen chauffeur) and carry them to lymph nodes. Lymph nodes full of nommy, nommy CD4+ T-cells.
Turns out thats another thing HIV-1 and HTLV have in common! They need dendritic cells to shuttle them around. But HTLV takes it to an extreme– the viruses cant infect CD4+ T cells on their own. They must be picked up by a dendritic cell and presented to a CD4+ T cell via cell-cell contact to infect their target cell. After that CD4+ T cell is infected, it can go on to infect other CD4+ T cells (again, through cell-cell contact, not free virus).
HTLV also causes immunosuppression, like HIV-1, but its not through slaughtering all your CD4+ T cells! HTLV use a different mind control trick to force the infected cell to produce too many cytokines, preventing the production of antibodies, meaning bacterial/fungal/parasitic infections can take you down.
There is a lesson to be learned from HTLV.
You dont need a million genes in a huge genome with epigenetics and siRNA and lots of crazy stuff to accomplish a lot.
All you really need is mind control.