To enter target cells, HIV-1 requires both CD4 and a coreceptor, predominantly CCR5. Blocking of the preferentially used CCR5 receptor by inhibitors or through gene knockdown conferred antiviral protection to R5-tropic variants.13,14 The homozygous CCR5 delta32 deletion, observed in approximately 1% of the white population, offers a natural resistance to HIV acquisition. We report a successful transplantation of allogeneic stem cells homozygous for the CCR5 delta32 allele to a patient with HIV.

Although discontinuation of antiretroviral therapy typically leads to a rapid rebound of HIV load within weeks, in this patient, no active, replicating HIV could be detected 20 months after HAART had been discontinued.15 This observation is remarkable because homozygosity for CCR5 delta32 is associated with high but not complete resistance to HIV-1. This outcome can be explained by the behavior of non-CCR5-tropic variants, such as CXCR4-tropic viruses (X4), which are able to use CXCR4 as a coreceptor. The switch occurs in the natural course of infection, and the proportion of X4 increases with ongoing HAART.16 Genotypic and phenotypic assays can be used to determine the nature and extent of coreceptor use, but the presence of heterogeneous viral populations in samples from patients limits the sensitivity of the assay.17 When genotypic analysis was performed in two laboratories applying WebPSSM and geno2pheno prediction algorithms, X4 variants were not detected in the plasma of our patient. To determine the proportion of minor variants in the plasma, we performed an ultradeep sequencing analysis, which revealed a small proportion of X4 variants before the allogeneic stem-cell transplantation.

Even after prolonged HAART, the persistence of HIV-1 populations in various anatomical compartments can be observed in patients without detectable viremia.18 In particular, the intestinal lamina propria represents an important reservoir of HIV-1, and genomic virus detection is possible in patients without viremia.19 In this patient, a rectal biopsy performed 159 days after transplantation revealed that CCR5-expressing macrophages were still present in the intestinal mucosa, indicating that they had not yet been replaced by the new immune system. Although these long-lasting cells from the host can represent viral reservoirs even after transplantation, HIV-1 DNA could not be detected in this patient's rectal mucosa.

It is likely that X4 variants remained in other anatomical reservoirs as potential sources for reemerging viruses, but the number of X4-tropic infectious particles after transplantation could have been too low to allow reseeding of the patient's replaced immune system.

The loss of anti-HIV, virus-specific, interferon-γ–producing T-cells during follow-up suggests that HIV antigen stimulation was not present after transplantation. This disappearance of effector T cells was not associated with a deficient immune reconstitution, as shown by the absence of relevant infection or reactivation of other persistent viruses, such as CMV and Epstein–Barr virus. Thus, the absence of measurable HIV viremia in our patient probably represents the removal of the HIV immunologic stimulus.20 Antibodies against HIV-envelope antigens have remained detectable, but at continually decreasing levels. The sustained secretion of antibodies might be caused by long-lived plasma cells that are relatively resistant to common immunosuppressive therapies.21,22

In the past, there were several attempts to control HIV-1 infection by means of allogeneic stem-cell transplantation without regard to the donor's CCR5 delta32 status, but these efforts were not successful.23 In our patient, transplantation led to complete chimerism, and the patient's peripheral-blood monocytes changed from a heterozygous to a homozygous genotype regarding the CCR5 delta32 allele. Although the patient had non–CCR5-tropic X4 variants and HAART was discontinued for more than 20 months, HIV-1 virus could not be detected in peripheral blood, bone marrow, or rectal mucosa, as assessed with RNA and proviral DNA PCR assays. For as long as the viral load continues to be undetectable, this patient will not require antiretroviral therapy. Our findings underscore the central role of the CCR5 receptor during HIV-1 infection and disease progression and should encourage further investigation of the development of CCR5-targeted treatment options.