In this prospective, longitudinal analysis of the impact of HIV infection and ART therapy on CD4+
T cell populations in both the inductive and effector sites of GALT, peripheral blood, and LNs, we provide quantitative measures of the extent of depletion and reconstitution in each compartment for the same individual. Importantly, we show that in the earliest stages of infection, CD4+
cell populations have been depleted in LN tissue, Peyer patches, and the lamina propria to an extent not evident in peripheral blood, such that when the patient presents with symptoms of acute HIV sero-conversion, the population of CD4+
cells in GALT and secondary LN tissue is already reduced by approximately 50%, compared with that in Peyer patches, LN tissue, or peripheral blood compartments. This result is in agreement with those of other studies that examined the lamina propria in contrast with the Peyer patches, LN, or peripheral blood compartments [6
We also quantified the differential impact of ART on CD4+
cell reconstitution in peripheral blood, compared with the other compartments. After 6 months of ART, the population of peripheral blood CD4+
cells had increased 41%, whereas the CD4+
cells in LN tissue increased only 15%, those in Peyer patches decreased by 22%, and those in the lamina propria decreased by 10%. We show, by combining flow cytometric analysis of cell suspensions with immunohistochemistry and quantitative image analysis, that this decrease occurs in EM cells, the major CD4+
cell subset population in the lamina propria. The reasons why the EM population in the lamina propria decreased in size are likely complex, but ongoing responses to microbes in the gut [23
], frequent reactivation of a latent mucosal infections such as HSV2 [24
], and/or a possible response to continuing low level of HIV replication despite ART could all contribute to the continuing drain on this population relative to naive and CM cells, which are also compromised, as discussed below.
The major new finding in this study is the early and extensive collagen deposition in GALT, to a greater extent than that which occurs in secondary lymphatic tissues, which we show to be correlated with greater depletion and limited reconstitution of CD4+
cells in the gut. We have previously shown in peripheral lymphatic tissues that fibrosis in the T cell zone is associated with reduced numbers of CD4+
cells and the potential for reconstituting naive CD4+
cells in particular, by a mechanism we refer to as the damaged niche, in which fibrosis disrupts the lymphatic tissue architecture and compromises the ability of lymphatic tissues to support T cell survival and proliferation [12
]. Of interest, levels of collagen in GALT do not predict immune reconstitution in peripheral blood. We think that the rapid and more extensive collagen deposition in the gut may be an important mechanism that contributes to the disproportionate levels of early and sustained depletion of CD4+
cells in GALT.
Under this model, early fibrotic damage to the Peyer patches contributes to the depletion and limits the reconstitution of the naive and CM CD4+
T cell populations; it also contributes to the sustained depletion of EM cells in the lamina propria by compromising the source of these cells in the face of continuing drains on the population, as described above. Prior to initiation of ART, the relationship between fibrotic damage to the inductive source and EM CD4+
T cell populations in the lamina propria may be important in determining rates of progression to disease, based on studies of SIV infection in the nonhuman primate model [28
]; this relationship may also be important in explaining clinical benefit despite apparent small increases in reconstitution. In SIV infection of rhesus macaques, there is massive depletion of memory CD4+
cells in early infection, mainly in the lamina propria, and there is little recovery with treatment; the greatest reconstitution occurs if therapy is initiated in the early stage of infection. Picker et al. [30
] have shown that the best predictor of disease progression in the SIV–rhesus macaque model is preservation of CM CD4+
cells, which presumably can continue to supply sufficient numbers of cells at effector sites to defend the host against pathogens in the gut and elsewhere. Similarly, less fibrotic damage and relatively greater preservation of an inductive source— here shown to include naive CD4+
cells—may better balance continuing drains on EM cells in the gut and produce clinical benefit despite apparently small increases in CD4+
We further confirm that ART initiated at later stages of HIV-1 infection does not result in gut reconstitution, but we provide new evidence in support of the conclusion that reconstitution of subpopulations of gut CD4+ cells is possible with early treatment, in agreement with some— but not most—recent studies. What we show here is that reconstitution is inductive site specific and subset specific, which may be the explanation for the discrepancy between our results and those obtained in these previous studies. Moreover, we show that early initiation of ART supports the greatest reconstitution of this population, as well as that of the peripheral blood and peripheral lymphatic tissues.
The benefits of early treatment for immune reconstitution—especially in the gut, the largest lymphoid organ of the immune system–suggests to us that current recommendations to wait for a CD4+
T cell count of 350 cells/mm3
before initiation of ART may not be optimal for the restoration of immunity to the extent that may prove to be necessary for immunosurveillance against tumors and pathogens over a relatively normal life span. Although current guidelines support the use of ART if the patient is diagnosed in the acute stage of disease, they are silent with respect to duration of therapy; some advocate stopping therapy in the chronic stage of disease. The strategy of interrupted therapy was recently tested in a large, international trial and was discontinued early as it was associated with more rapid progression of HIV infection and worse clinical outcome [31
]. Our observations suggest that earlier therapy might better preserve and restore the critical CM CD4+
cell population in GALT. We also speculate that antifibrotic drugs might have a role as adjunctive therapy in HIV-1 infection, both in limiting depletion and improving reconstitution during ART.