We investigated the interplay occurring between pathogens in the course of dual infections, using an in vitro model in which the THP-1 monocytic cell line is first infected with HSV-1 and then exposed to Ca or Cn. These three pathogens share some pathogenic features: they cause opportunistic infections, target macrophages and are neurotropic. Here, we show that HSV-1-infected THP-1 cells exhibited augmented phagocytosis againstthetwo opportunisticfungi butreduced capability to counteractfungal infection: the better ingestion by monocytes was followed by facilitated fungal survival and replication. Reduced IL-12 production was also observed. Cytofluorimetric analysis showed that HSV-1-infected monocytes exhibit: (i) downregulated TLR-2 and TLR-4, critical structures in fungal recognition; (ii) reduced expression of CD38 and CD69, known to be important markers of monocyte activation; and (iii) enhanced expression of apoptosis and necrosis markers, in the absence of altered cell proliferation. Overall, these findings imply that HSV-1 infection prevents monocyte activation, thus leading to a significant dysfunction of the monocyte-mediated anti-Candida response; HSV-1 induced apoptosis and necrosis of monocytes further contribute to this impairment.

In order to investigate the interplay occurring between pathogens in the course of double infections, an in vitro model was set up in which the monocytic cell line THP-1 was exposed to Cryptococcus neoformans (Cn) and human herpesvirus 6 (HHV-6). Cn and HHV-6, both highly neurotropic, can cause serious diseases of the central nervous system and have monocytes, among other cell types, as target cells, causing alteration of their secretion pattern. Here, it was shown that unlike THP-1 cells exposed to cell-free virus inocula, THP-1 exposed to HHV-6-producing lymphocytes exhibited augmented phagocytosis against Cn. The phenomenon occurred after 24 h of monocyte/lymphocyte co-culture and was independent of direct cell-to-cell contact. Moreover, in the presence of HHV-6, THP-1 cells expressed enhanced secretory responses but reduced capability to counteract fungal infection: the enhanced ingestion by monocytes was followed by facilitated fungal survival and replication. These data provide initial in vitro evidence that HHV-6 may dysregulate monocyte-mediated anticryptococcal defences with an overall pro-cryptococcus result.

1. Microglial cell activation occurs during brain injury, ischemia, and in several neurologic disorders. Recently, we isolated a transmissible cytotoxic activity (TCA) from the cerebrospinal fluid of a patient with brain ischemia. Such a TCA, associated with one or more protein(s) that supposedly had undergone in vivo misfolding, causes apoptosis in vitro in different cell lines, including microglial cells. The TCA producing cells and the potential in vivo role of such cytotoxic activity remains to be elucidated. Here, we investigated the in vitro effects of TCA on microglial cell immune functions.2. The murine microglial cell line RR4 was exposed to TCA, and then its response was evaluated as: (a) phagocytosis and antifungal activity against Candida albicans; (b) secretory pattern; and (c) levels of p38 phosphorylation.3. Unlike mock-treated controls, microglial cells exposed to TCA showed an increase in phagocytic activity. Unexpectedly, their capability to kill the ingested fungi significantly diminished. Moreover, TCA-treated cells produced amounts of macrophage inflammatory protein 1-alpha, tumor necrosis factor-alpha, and nitric oxide significantly higher than mock-treated cells. Finally, phosphorylation of p38 mitogen-activated protein kinase (MAPK) was detected in TCA-treated but not in mock-treated controls as early as 30 min after treatment.4. Overall, these results indicate that TCA causes a rapid molecular response in microglial cells, by the time, leading to an intriguing effector and secretory dysfunction.

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