Prior to the development of effective treatments for HIV infection, the overwhelming majority of HIV-infected patients experienced an inexorable depletion of CD4 T cells, resulting first in immunodeficiency, followed by development of one or more AIDS defining illnesses and then death. The advent of effective therapies, and the knowledge of how best to use them, led to a reversal of this pathway for a majority of patients. Following initiation, the desired response to antiretroviral therapy is a reduction in the level of viral replication, reflected by reduced plasma viremia, and an increase in CD4 T cell number.
The study of exceptional circumstances has lead to some of the most valuable advances in our understanding of HIV pathogenesis. For example, studying a subset of long term non-progressors identified the CCR5Δ32 mutation as a factor in disease progression, which in turn has lead to the development of chemokine receptor antagonists as therapies for HIV infection. Another exceptional circumstance in HIV infection is the situation of immunologic response and virologic non response to antiretroviral treatment. Such discordance has been intensely studied and found to occur in 10–20% of cases of patients initiating therapy 
and is more likely to occur in patients receiving a protease inhibitor 
. Immunologically, discordant patients have reduced rates of apoptosis of CD4 T cells 
and higher TREC content 
, suggesting improved thymic function. Other quantitative improvements in immune function have also been observed relative to patients with both immunologic and virologic failure: improved lymphoproliferative responses to recall antigens and HIV antigens, improved cytokine production and improved IFNγ production 
. Of importance, such responses can be maintained for as long as five years 
, and are associated with a reduced risk of disease progression 
and death 
compared to non-responders. Despite few mechanistic studies aimed at understanding such discordance, it is accepted that in the presence of reduced drug susceptibility, a portion of benefit achieved from therapy is due to persistent antiviral activity, as well as reduced replicative capacity 
. Of course this contention requires that reduced replicative capacity translates into reduced CD4 T cell losses, although this has never been specifically demonstrated.
Expression of HIV-1 protease in cells is intrinsically cytotoxic and this property was exploited to screen and develop the protease inhibitor class, which has become an important treatment for HIV-1 infected patients. Indeed, HIV protease has degenerate substrate specificity and various host cellular proteins are targets for HIV-1 protease which are then cleaved following HIV infection including: actin, macroglobulin, myosin, spectrin, vimentin, desmin, DNA fragmentation factors, filamin, eukaryotic translation initiation factor-4γ, fibronectin, microtubule–associated proteins and other proteins 
. Since HIV protease is active in the cytoplasmic compartment of HIV-infected cells, the cytotoxic effects are directly or indirectly related to cleavage of one or more of these cellular proteins 
. One substrate of HIV protease is procaspase 8, and the absence of procaspase 8 renders cells resistant to the cytotoxic effects of HIV protease 
. Specifically at a molecular level, HIV protease cleaves procaspase 8 between two phenylalanine residues, F355 and F356, generating a novel caspase 8 fragment of 41 kd that we call Casp8p41 
. Expression of Casp8p41 is independently cytotoxic and induces a similar death pathway as HIV protease: mitochondrial depolarization, release of cytochrome c, activation of caspases 9 and 3, and DNA fragmentation 
. Importantly, generation of Casp8p41 occurs only following HIV infection or ectopic expression of HIV protease and not following death induced by other HIV proteins, death receptor signaling or mitochondrial toxins 
. Consequently, detecting Casp8p41 in cells from HIV-infected patients 
and the ability of Casp8p41 levels to predict CD4 T cell losses 
, altogether suggests that HIV protease-mediated killing occurs in vivo
, and is of direct relevance to HIV pathogens. Further, lymph nodes from HIV-infected patients with detectable viral replication also express Casp8p41, which colocalizes with both infected and apoptotic cells 
. Altogether, therefore, HIV protease production of Casp8p41 is one mechanism by which HIV-infected T cells can die, that may contribute to the overall CD4 T cell loss which occurs in vivo
. In the current study, we evaluated whether protease mutations which occur in patients with discordant antiretroviral responses differ in their ability to generate Casp8p41 and to cause cell death, compared to patients with concordant antiretroviral responses.