A murine model of dual infection with cytomegalovirus and Pneumocystis carinii: effects of virus-induced immunomodulation on disease progression.

Despite the use of antimicrobial prophylaxis, cytomegalovirus (CMV) and Pneumocystis carinii (PC) pneumonia (PCP) are both leading causes of morbidity and mortality in immunocompromised patients. It has previously been reported that CMV infection modulates host immune responses with a variety of mechanisms which include the suppression of helper T cell functions and antigen presenting cell (APC) functions, both of which are critical for PCP resolution. However, the mechanisms of these interactions and other possible immune regulatory effects are not clearly understood. In this study, we investigated the impact of murine CMV (MCMV) induced immunomodulation on the progression of PCP in a co-infection model. Initial results show that dually infected mice had evidence of more severe PC disease, which include a greater loss of body weight, an excess lung PC burden and delayed clearance of PC from lungs, compared to mice with PC infection alone. At day 7 post-infection, dually infected mice had reduced numbers of MHC-II expressing cells in the lung interstitium and lymph nodes and reduced migration of CD11c+ cells to both the tracheobronchial lymph nodes and alveolar spaces. Dual infected mice showed elevated numbers of specific CD8 responses concomitant with a decrease in activated CD4+ T cells in both the lymph nodes and in alveolar spaces when compared to mice infected with MCMV alone. These data suggest that MCMV infection inhibits the immune responses generated against PC which contribute to the delayed clearance of the organism.

Modulation of proinflammatory responses to Pneumocystis carinii f. sp. muris in neonatal mice by granulocyte-macrophage colony-stimulating factor and IL-4: role of APCs.

Clearance of Pneumocystis carinii f. sp. muris (PC) organisms from the lungs of neonatal mice is delayed due to failure of initiation of inflammation over the first 3 wk after infection. The ability of neonatal lung CD11c(+) dendritic cells (DCs) to induce Ag-specific T cell proliferative responses was significantly reduced compared with adult lung DCs. However, neonatal bone marrow-derived DCs were as competent at presenting PC Ag as were adult bone marrow-derived DCs. Because GM-CSF mRNA expression and activity were significantly reduced in neonatal lungs compared with adults, we treated neonates with exogenous GM-CSF and IL-4 and found enhanced clearance of PC compared with untreated neonates. This was associated with increased lung TNF-alpha, IL-12p35, and IL-18 mRNA expression, indicating enhanced innate immune responses. Cytokine-treated mice had marked expansion of CD11c(+) DCs with up-regulated MHC-II in the lungs. Moreover, increased numbers of activated CD4(+)CD44(high)CD62L(low) cells in the lungs and draining lymph nodes suggested improved Ag presentation by the APCs. Together these data indicate that neonatal lungs lack maturation factors for efficient cellular functioning, including APC maturation.

TNF-alpha-dependent ICAM-1- and VCAM-1-mediated inflammatory responses are delayed in neonatal mice infected with Pneumocystis carinii.

Neonatal mice have a delayed CD4-mediated inflammatory response to Pneumocystis carinii (PC) infection in the lungs that corresponds to a delayed TNF-alpha response and a delayed clearance of the organisms compared with adult mice. Since TNF-alpha is known to drive the up-regulation of adhesion molecules, we examined the expression and function of adhesion molecules in the lungs of neonatal mice. The expression of both ICAM-1 and VCAM-1 was significantly lower in the lungs of PC-infected neonatal mice compared with adults. Additionally, migration of neonatal T cells across endothelial cells expressing VCAM-1 and monocyte chemotactic protein-1 was aberrant compared with that in adult T cells, although alpha(4)beta(1) integrin-mediated adhesion of neonatal lymphocytes was comparable to that of adult lymphocytes. Treatment of neonatal mice with exogenous TNF-alpha resulted in increased expression of ICAM-1 and VCAM-1 as well as increased expression of chemokines, resulting in infiltration of CD4(+) cells into the lungs. Treatment with exogenous TNF-alpha resulted in a trend (although not statistically significant) toward a reduction of PC organisms from the lungs. These data indicate that neonatal lung endothelial cells do not up-regulate ICAM-1 and VCAM-1 in response to PC infection, probably due to depressed TNF-alpha production. Additionally, neonatal T cells are defective in the ability to migrate across endothelial cells.

IL-10 modulates host responses and lung damage induced by Pneumocystis carinii infection.

Host responses to Pneumocystis carinii infection mediate impairment of pulmonary function and contribute to the pathogenesis of pneumonia. IL-10 is known to inhibit inflammation and reduce the severity of pathology caused by a number of infectious organisms. In the present studies, IL-10-deficient (IL-10 knockout (KO)) mice were infected with P. carinii to determine whether the severity of pathogenesis and the efficiency of clearance of the organisms could be altered in the absence of IL-10. The clearance kinetics of P. carinii from IL-10 KO mice was significantly enhanced compared with that of wild-type (WT) mice. This corresponded to a more intense CD4(+) and CD8(+) T cell response as well as an earlier neutrophil response in the lungs of IL-10 KO mice. Furthermore, IL-12, IL-18, and IFN-gamma were found in the bronchoalveolar lavage fluids at earlier time points in IL-10 KO mice suggesting that alveolar macrophages were activated earlier than in WT mice. However, when CD4(+) cells were depleted from P. carinii-infected IL-10 KO mice, the ability to enhance clearance was lost. Furthermore, CD4-depleted IL-10 KO mice had significantly more lung injury than CD4-depleted WT mice even though the intensity of the inflammatory responses was similar. This was characterized by increased vascular leakage, decreased oxygenation, and decreased arterial pH. These data indicate that IL-10 down-regulates the immune response to P. carinii in WT mice; however, in the absence of CD4(+) T cells, IL-10 plays a critical role in controlling lung damage independent of modulating the inflammatory response.