The role of lympho-epithelial interactions in the regulation of small intestinal epithelium proliferation

Cryptosporidiosis in rhesus macaques challenged during acute and chronic phases of SIV infection

The intestinal immune dysfunction due to loss of mucosal and peripheral CD4(+) T cells in individuals with HIV/AIDS is presumably responsible for the establishment of persistent cryptosporidiosis. Simian immunodeficiency virus (SIV)-infected macaques were used to investigate the phase/timing in SIV infection, which permits a self-limiting Cryptosporidium parvum infection to become persistent in immunodeficient hosts because of significant mucosal immune defects. Two groups of SIV-infected macaques were challenged with C. parvum; one was challenged during the acute SIV infection phase (2 weeks post-SIV infection) and the second was challenged during the chronic SIV phase (CD4 counts 200-500 cells/μl of blood). Samples (fecal, blood, biopsy, and necropsy) were collected at different time points after infection to correlate the progression of disease with the immune status of the animals. All seven SIV-infected macaques challenged during the acute phase of SIV infection became persistently infected and excreted oocysts for 1-4 months. However, four of the six in the chronic SIV phase became infected with cryptosporidiosis, of which one survived 2 weeks and one became naturally infected. Sequential analysis of CD4(+) in blood and intestines of coinfected macaques exhibited pronounced losses of CD4 T cells during the first 2 weeks after SIV infection, followed by transient rebound of CD4 T cells in the gut after C. parvum infection, and then a gradual loss over subsequent months. Persistent cryptosporidiosis was more consistently induced during the acute SIV phase indicating that profound viral damage to gut lymphoid tissue during the acute phase was more conducive, compared with the chronic phase, to establishing persistent cryptosporidiosis than low circulating CD4 T cells.

The mucosal immune system: primary target for HIV infection and AIDS

Despite intensive research, several questions remain regarding the pathogenesis of infection with HIV-1. Recently, it has been shown that simian immunodeficiency virus (SIV) selectively targets and destroys specific subsets of CD4+ T cells that are abundant in mucosal tissues but rare in peripheral lymphoid tissues. This finding could be highly relevant in explaining a major paradox in the infection and elimination of CD4+ T cells during HIV infection: the progressive decline in the number of CD4+ T cells in the blood, despite the paucity of HIV-infected cells in this tissue. This article discusses the hypothesis that infection with HIV and SIV, and the resulting disease, is governed by the state of cellular activation and the expression of chemokine receptors by specific subsets of CD4+ T cells residing in mucosal lymphoid tissues, rather than those found in the peripheral blood or lymph nodes.