Background:  African green monkeys infected by SIV_agm do not develop AIDS. This lack of disease is associated with low RNA and DNA viral loads in lymph nodes despite a high plasma viremia. In order to investigate the role of dendritic cells in SIV_agm dissemination to lymph nodes, we analyzed here the capacity of African green monkey dendritic cells to transmit SIV_agm to T cells. Furthemore we evaluated the expression levels and virological functions of African green monkey DC-SIGN.

Methods:  Expression of DC-SIGN mRNA in lymph node cells was searched by RT-PCR . The mRNA isoforms were cloned and sequenced. Transient expression on cell lines allowed the identification of monoclonal antibody recognizing the African green monkey molecule. Expression of DC-SIGN on lymph node cells was determined by immunohistochemistry. Non permissive CD4⁺CCR5^- HeLa cells transiently expressing DC-SIGNagm were exposed to SIV_agm and co-cultured with T-cells to assess infection in trans. Enhancement of infection in cis was assessed by exposure of CD4⁺CCR5⁺ cells to SIV_agm. Myeloid dendritric cells were derived from monocytes (MDDC) in the presence of GM-CSF and IL-4. The numbers of DC-SIGN molecules expressed on African green monkeys and human MDDC were compared. The capacity of African green monkey MDDC from 10 distinct animals to transmit SIV_agm to T cells was evaluated and the implication of DC-SIGN tested in inhibition assays using anti-DC-SIGN monoclonal antibodies and mannan in the presence or not of AZT.

Results:  We easily detected full-length DC-SIGN mRNA in lymph node cells. Furthermore, we observed large cells expressing DC-SIGN at their surface in the medulla and paracortex of African green monkey lymph nodes. The predominant dc-sign mRNA isoform (DC-SIGN_agm1) differed by 3 to 7 mutations as compared to DC-SIGN alleles described in macaques. DC-SIGN_agm1 allowed transmission of SIV_agm to T-cells and enhanced cellular entry of SIV_agm when expressed in cis. Furthermore, African green monkey MDDC expressed at least 100 000 DC-SIGN molecules at their surface and efficiently transmitted SIV_agm to T cells. At low moi (10-5 TCID50/cell) viral transmission was mainly DC-SIGN dependent.

Conclusions:  This study highlights the role of DC-SIGN in the presence of low levels of infectious virus. Furthermore, our data indicate that the virus transmission capacity of DC-SIGN is not associated with levels of viral load in lymph nodes. In conclusion, additional factors, such as the frequency in lymlph nodes of major target cells (activated T CD4⁺ lymphocytes), are more likely determinating factors for viral replication levels in lymph node.

Keywords: dendritic cell; SIV; DC-SIGN