Retroviruses: HIV Replication and Immune Destruction, and HTLV

Following gp41-mediated membrane fusion and release of the viral capsid into the cytoplasm, the capsid partially uncoats, and reverse transcriptase begins copying the diploid RNA genome into a single double-stranded DNA copy.

This process is genuinely error-prone, since reverse transcriptase lacks proofreading capability, directly generating the extensive population-level genetic diversity, quasispecies, that underlies HIV's capacity for rapid immune escape and antiretroviral drug resistance development.

The diploid genome arrangement also permits genuine recombination between the two RNA copies packaged within a single virion. This is particularly significant when that virion was assembled in a cell co-infected by two genetically distinct HIV variants. It adds a further, distinct mechanism of genetic diversification beyond point mutation alone.

The newly synthesised double-stranded DNA copy is transported into the host cell nucleus and inserted into the host chromosome by integrase, forming the provirus.

Once integrated, the provirus persists for the lifetime of that host cell, and any daughter cells produced if the host cell itself divides. Critically, it can remain transcriptionally latent within long-lived resting memory CD4 T cells, producing no viral protein. It is therefore invisible to both immune surveillance and to antiretroviral drugs, which act only on actively replicating virus, not on integrated, transcriptionally silent proviral DNA.

This latent reservoir is the fundamental biological reason current antiretroviral therapy, despite being highly effective at suppressing active viral replication, does not achieve a true cure, and is the central target of ongoing HIV cure research strategies.

Following integration, the provirus can be transcribed, using host RNA polymerase II, to produce new viral genomic RNA and mRNA for viral protein synthesis. This step is regulated by both host cell activation state and specific virus-encoded regulatory proteins. Actively dividing, activated CD4 T cells support much more efficient proviral transcription than resting cells, which is directly relevant to viral latency.

Newly synthesised genomic RNA and structural proteins assemble at the plasma membrane, and immature virions bud outward, acquiring their envelope from the host plasma membrane in the process, before undergoing the protease-mediated maturation step that renders them fully infectious.

HIV tropism evolves over the untreated natural history of infection in a proportion of patients. R5, CCR5-using, macrophage-tropic, strains are dominant during early and chronic infection. X4, CXCR4-using, T-cell-tropic, strains emerge in a subset of patients later in the disease course, associated with accelerated CD4 decline.

Beyond CD4 T cells, HIV also infects macrophages and dendritic cells, at lower CD4 density but still sufficient for productive or latent infection. Macrophages, given their relative resistance to the cytopathic effects that kill infected CD4 T cells directly, are believed to function as an additional, long-lived viral reservoir. Dendritic cells, via their normal antigen-presenting function, can also facilitate viral transmission to CD4 T cells within lymphoid tissue.

The defining pathogenic consequence of HIV infection is progressive, profound depletion of the CD4 T helper lymphocyte population, via a combination of mechanisms:

• direct viral cytopathic killing of productively infected cells
• cytotoxic T lymphocyte-mediated killing of infected cells, the same clearance-versus-damage double-edged mechanism seen throughout viral pathogenesis
• chronic, generalised immune activation and inflammation

The immune activation and inflammation are themselves at least partly driven by microbial translocation across a progressively damaged gastrointestinal mucosal barrier. This is an increasingly recognised contributor to the overall pathogenic process beyond direct viral cytotoxicity alone.

Because CD4 T helper cells are themselves essential coordinators of both humoral, antibody, and cell-mediated adaptive immune responses, their progressive depletion produces a correspondingly progressive, generalised impairment of the host's capacity to mount effective immune responses against essentially any pathogen, not solely HIV itself. This directly explains the extraordinarily broad range of opportunistic infections and malignancies characteristic of advanced, untreated HIV infection, AIDS.

The CD4 count itself, reflecting the cumulative extent of this depletion, is the principal laboratory marker of HIV disease stage and the basis for opportunistic infection risk stratification used throughout clinical HIV management.

HTLV-1

Genome: positive-sense ssRNA, retroviral. Envelope: present.

HTLV-1 shares the fundamental retroviral replication strategy, reverse transcriptase, integrase, provirus formation, but with a critically different clinical consequence. Rather than causing progressive immune cell depletion and immunodeficiency as HIV does, HTLV-1 causes malignant transformation of infected T lymphocytes in a proportion of infected individuals, reflecting a fundamentally different, oncogenic rather than cytopathic or immunodepleting pathogenic mechanism despite the shared basic retroviral biology.

Transmission occurs through breastfeeding, a particularly efficient and epidemiologically important route given prolonged exposure, sexual contact, and blood or blood product transfusion. The virus is geographically concentrated in specific endemic regions, notably southwestern Japan, the Caribbean, and parts of Africa and South America, with seroprevalence and clinical disease incidence substantially lower outside these regions.

Oncogenic association

Adult T-cell leukaemia and lymphoma occurs in a minority of infected individuals, generally cited around two to five percent lifetime risk, typically after a prolonged latency period of decades following initial infection, often acquired in infancy via breastfeeding, with malignancy presenting only in mid-to-late adulthood. The underlying oncogenic mechanism involves the HTLV-1 Tax protein directly promoting infected T cell proliferation and genomic instability, a mechanism distinct from the DNA-tumour-virus mechanisms of viruses such as HPV or EBV.

HTLV-1 is also associated with HTLV-1-associated myelopathy and tropical spastic paraparesis, a chronic, progressive spastic paraparesis reflecting inflammatory, immune-mediated spinal cord involvement rather than direct malignant transformation.

This illustrates that a single virus can produce genuinely distinct pathogenic outcomes, malignant versus inflammatory and autoimmune-pattern, in different affected individuals. It is a point worth holding alongside HIV's comparatively more uniform pathogenic mechanism, progressive CD4 depletion, across essentially all infected individuals.