Summary While the signals that control neutrophil migration from the blood to sites of infection have been well characterized, little is known about their migration patterns within lymph nodes, or the strategies that neutrophils use to find their local sites of action. To address these questions, we used two-photon scanning laser microscopy (TPSLM) to examine neutrophil migration in intact lymph nodes during infection with an intracellular parasite, Toxoplasma gondii. We find that neutrophils form both small, transient or large, persistent swarms via a strikingly coordinated migration pattern. We provide evidence that cooperative action of neutrophils and parasite egress from host cells can trigger swarm formation. Neutrophil swarm formation coincides in space and time with the removal of macrophages that line the subcapsular sinus of the lymph node. Our data provide insights into the cellular mechanisms underlying neutrophil swarming and suggest new roles for neutrophils in shaping immune responses.

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Endogenous retroviruses (ERVs) are the remnants of ancient retroviral infections of germ cells and have been maintained in whole or part as heritable genomic elements. The last known endogenization events occurred several million years ago, and therefore stepwise analysis of retroviral endogenization has not been possible. A unique opportunity to study this process became available when a full-length ERV isolated from koalas (KoRV) was shown to have integrated into their germ line within the past 100 years. Even though KoRV shares 78% nucleotide identity with the exogenous and highly infectious gibbon ape leukemia virus (GALV), the infectivity of KoRV, like that of other ERVs, is substantially lower than that of GALV. Differences in the protein coding regions of KoRV that distinguish it from GALV were introduced into the GALV genome, and their functional consequences were assessed. We identified a KoRV gagpol L domain mutation as well as five residues present in the KoRV envelope (env) that, when substituted for the corresponding residues of GALV, resulted in vectors exhibiting substantially reduced titers similar to those observed with KoRV vectors. In addition, KoRV env protein lacks an intact CETTG motif that we have identified as invariant among highly infectious gammaretroviruses. Disruption of this motif in GALV results in vectors with reduced syncytia forming capabilities. Functional assessment of specific sequences that contribute to KoRV's attenuation from a highly infectious GALV-like progenitor virus has allowed the identification of specific modifications in the KoRV genome that correlate with its endogenization.adaptation ͉ endogenous retrovirus ͉ koala K oala retrovirus (KoRV) infection is widespread among koalas of mainland Australia. In the early 1920s, a founder population of koalas from the southeastern state of Victoria was established on Kangaroo Island. The Kangaroo Island koalas were recently reported free of KoRV. The discovery of a KoRV-free population of koalas, together with the observations that KoRV remains actively transcribed in its host and that KoRV has integrated into germ line tissue, suggests that this retrovirus is a recently introduced endogenous retrovirus (ERV) (1). KoRV's closest genomic relative is the exogenous gibbon ape leukemia virus (GALV). The only recorded GALV outbreak was confined to gibbon apes originating from an animal holding facility in Thailand in the late 1960s through the 1970s (2). In contrast, since the initial observation of leukemias and neoplasias in koalas in the 1960s (3) and the description of a gammaretrovirus as the possible cause (4), KoRV is now recognized as endemic among Australian mainland koalas (1, 5).Although ERVs are for the most part dormant, there is an increasing amount of evidence to suggest that mobile retroelements contribute to genomic evolution (6, 7). KoRV is unusual in that it coexists as both an exogenous and endogenizing viral agent, providing a very rare real-time model for the role, if any, of viral endogenization in speci...

Toxoplasma gondii (Toxo) can cause severe disease in immunocompromised individuals. Previous studies in various strains of mice have shown that genetic resistance to disease is linked to the MHC locus and protection is largely mediated by Toxo-specific CD8 T cells. In B10.D2 (H-2d) mice, CD8 T cells recognize an immunodominant peptide derived from the Toxo-protein, GRA6. However, a GRA6-specific response is not detected in C57BL/6 (B6, H-2b) mice. Instead, Toxo immunization of B6 mice elicits primarily a CD4 T cell response to unknown Toxo antigens. To identify the CD4 T cell stimulating antigens, we generated a Toxo-specific, lacZ inducible, CD4 T cell hybridoma and used it as a probe to screen a Toxo-cDNA library in bone marrow derived dendritic cells. The results reveal that CD4 T cells can be elicited by a hypothetical protein in the Toxo sequence database. We have defined the minimal peptide presented by MHC class II molecules and we will discuss the protective potential of this peptide-specific CD4 T cells in Toxo infected mice.