Wednesday, December 15, 2010

Exciting prospects: Stem Cell Therapy in HIV therapy

I am tremendously psyched about this fascinating report published in Blood about a week back. The paper from this German multi-institutional group describes how, in an HIV-infected leukemic patient, transplantation with CCR5Δ32/Δ32 stem cells appeared to cure HIV. Even as I write this, I can barely contain my excitement; this finding has tremendous possibilities.

For the uninitiated, cellular chemokine receptors (CCR), as well as C-X-C Chemokine receptors (CXCR), are cell-surface protein molecules that ordinarily bind to chemokines (small protein molecules that act as chemo-attractants to guide the migration of cells for various purposes), activating signalling processes downstream into the cell. HIV, the human immunodeficiency virus, co-opts such receptors, most commonly CCR5 or CXCR4, to mediate its entry into its primary target, the CD4+ T-lymphocytes. Viral replication in these cells utilizes the cellular gene expression processes, which are, understandably, most active in activated cells; therefore, HIV infection targets activated CD4+ T-cells, especially the activated memory CD4+ T-cells in gastrointestinal mucosa, leading to their loss in the peripheral blood and lymphoid tissues, gradually destroying the immune system.

The aim of the currently available anti-retroviral therapies (ART, including 'Highly Active ART' or HAART) is to control the HIV replication, thereby allowing the immune system to be restored - which delays the disease progression. However, clever, little HIV establishes reservoirs of latently infected CD4+ T-cells and tissue phagocytes (such as macrophages and glial cells), as well as anatomical niches, such as the prostate gland, that continues to harbor the replication-competent virus despite ART - which allows for re-infection (technically, re-activation) once the ART is discontinued.

However, a multicenter cohort study, published in Science in 1996, made an amazing observation: Individuals whose cells contained two copies (homozygous) of a deletion variant (Δ32) of the CCR5 gene (CCR5Δ32/Δ32) - which abrogated cell surface expression of CCR5 - were naturally resistant to infection by CCR5-recognizing HIV strains. Moreover, progression to full blown AIDS was delayed in HIV-infected individuals by more than 16 years (the so-called 'Long Term Progressors') if they were homozygous for the Δ32 mutation, and by 2-4 years if they were heterozygous (i.e. had one copy of the mutated gene).

The German group conducting the current study had previously documented the absence of rebound viremia ("virus in blood") in the first 20 months after cessation of ART in their HIV-infected patient who received CCR5Δ32/Δ32 stem cells as treatment for relapsed acute myeloid leukemia (AML). This report raised the possibility of using transplantation of selected or transgenic hematopoietic stem cells as a treatment for HIV/AIDS.

However, in an already HIV-infected patient, ordinarily the pre-transplant conditioning (immune suppression, i.e. destruction of the transplant recipient's immune system in order to prevent rejection, as well as cytoreduction, i.e. the depletion of host cells - by means of intense chemotherapy and/or radiation) process doesn't lead to the complete elimination of HIV from the tissue reservoirs despite HAART, and such patients have been known to experience rebound viremia from the pre-transplant HIV population.

Therefore, in the current study, the German group looked for several parameters in their patient, namely:

(a) Immune reconstituion, i.e. the restoration of CD4+ T-cells in the patient. They evaluated the reconstitution in the systemic circulation, as well as in the mucosal immune system, for >3.5 years after the transplant.

(b) Continued HIV-susceptibility of the recovered immune cells. They analyzed the activation status and CXCR4 expression profile of the recovered CD4+ T-cells, as well as their susceptibility to a productive HIV infection.

(c) Stability and persistence of the latent HIV-reservoir during the period of immune reconstitution following the CCR5-mutated stem cell transfer. They examined different tissue compartments for donor (CCR5-negative) and host (CCR5-positive) immune cells by immuno-fluorescence microscopy as well as CCR5-genotyping.

This brings us to the most amazing part of their findings. They observed that:

  1. Host T-cells, including the long-lived HIV target cells of host-origin, were completely eliminated from the periphery following the transplant, being replaced by donor derived cells. This is an exceedingly important observation, since non-circulating immune cells (tissue T-cells and monocyte/macrophages) are virtually resistant to chemo/radio-therapy, and their elimination ensures removal of possible viral reservoirs. They found no CCR5 expressing host-origin cells in the liver, colon and brain within two years.
  2. Numbers of donor-derived peripheral CD4+ T cells increased continuously reaching healthy levels in two years, along with an enrichment of memory, as well as naïve, central, CD4+ T cells. Results from their control groups established that the T-cell recovery was primarily through the homeostatic proliferation of memory CD4+ T cells.
  3. Circulating donor-derived CCR5-negative CD4+ T cells were efficiently recruited to the gastrointestinal tract, repopulating the mucosal T-cell compartment.
  4. Over 45 months following transplantation, HIV RNA and DNA remained undetectable in tissue compartments, and HIV-specific antibodies in the serum gradually decreased over time.
  5. Importantly, however, the recovered, donor-derived CCR5-negative CD4+ T cells maintained the level of CXCR4 expression, thereby remaining vulnerable to HIV-variants that target this molecule (called 'X4 HIV'), which they also demonstrated experimentally.

Therefore, the CCR5Δ32/Δ32 stem cell transplantation in this HIV-infected AML patient was instrumental in successful recovery of CD4+ T cells and complete elimination of HIV, as well as HIV reservoirs, from the patient. This was likely possible because of their three-pronged approach: (a) The HAART was active against the virus, (b) the pre-transplant conditioning effectively reduced the peripheral HIV-infected cells, and (c) the stem-cell transplant effectively flushed out the remaining HIV-reservoirs, while remaining impervious to fresh infection.

As with any scientific study, this report, too, comes with caveats that temper the enthusiasm.A The most critical of such caveats is that these were observations in just a single patient. Secondly, the CCR5-negative CD4+ T cells, and thereby the patient, still remained susceptible to X4 HIV. This patient was possibly extremely fortunate to have been infected with the CCR5-tropic variant ('R5') of HIV, and not X4, and also not with a variant that mutated rapidly to lose its CCR5-tropicity. In addition, although by common prognostic markers (i.e. plasma viral load and peripheral CD4+ T cell count) shows absence of HIV-disease in this patient, presence of still-latent HIV in distinct tissue compartments, especially the hard-to-reach mucosal immune system, cannot still be completely discounted. Also important to remember is the fact that the bone-marrow transplant (hematopoietic stem cell transplant) process is inherently risk-laden, and still associated with significant mortality and morbidity.

But all in all, the fact that the stem-cell transplantation intended to treat AML has managed to keep this patient HIV-free for 3.5 years in absence of continued ART represents a powerful and exciting achievement in the continuous war against the scourge of HIV, opening up distinct possibilities for future HIV-therapy. One such possibility might be the voluntary, pro-active banking of autologous (from the same individual) stem cells, perhaps even with an engineered CCR5 mutation, that can be used later, in the event of an HIV-infection, or certain cancers. It appears that such banking services are already in operation, albeit currently of doubtful utility.

More reading:

1. Allers K, et al. Evidence for the cure of HIV infection by CCR5Δ32/Δ32 stem cell transplantation. Blood. 2010 Dec 8. [Epub ahead of print] PMID: 21148083 This is paper under discussion.

2. Chun TW, Fauci AS. Latent reservoirs of HIV: obstacles to the eradication of virus. Proc Natl Acad Sci U S A. 1999 Sep 28;96(20):10958-61. PMID: 10500107.

3. Smith DM, et al. The prostate as a reservoir for HIV-1. AIDS. 2004 Jul 23;18(11):1600-2. PMID: 15238781.

4. Dean M, et al. Genetic restriction of HIV-1 infection and progression to AIDS by a deletion allele of the CKR5 structural gene. Hemophilia Growth and Development Study, Multicenter AIDS Cohort Study, Multicenter Hemophilia Cohort Study, San Francisco City Cohort, ALIVE Study. Science. 1996 Sep 27;273(5283):1856-62. PMID: 8791590.

5. Hutter G, et al. Long-term control of HIV by CCR5 Δ32/Δ32 stem-cell transplantation. N Engl J Med. 2009;360(7):692-698. PMID: 19213682.

Note: A. A fact most purveyors of pseudoscience like to capitalize on, without understanding the dynamic, empiricism-driven, evidence-based nature of science.

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