Relating Retinal Vascular Oxygen Saturation and Microvasculature Morphology at Progressive Stages of Diabetic Retinopathy.

Purpose: Diabetic retinopathy (DR) is a common cause of vision loss in working age adults and presents changes in retinal vessel oxygenation and morphology. The purpose of this study was to test the hypothesis that there is an association of retinal vessel oxygen saturation with vessel density (VD) and tortuosity in DR. Methods: Ninety-five subjects were classified in the following groups: nondiabetic control (N = 25), no DR (N = 28), mild nonproliferative DR (NPDR; N = 21), moderate to severe NPDR (N = 14), or treated proliferative DR (PDR; N = 7). Retinal oximetry was performed to measure arterial and venous oxygen saturation (SO2A and SO2V) and calculate oxygen extraction fraction (OEF). Optical coherence tomography angiography (OCTA) was performed for measurements of VD and vessel tortuosity index (VTI). Results: There were statistically significant differences in SO2A and SO2V among groups (P ≤ 0.004). SO2A and SO2V were higher in the PDR group compared to the control group and SO2V was also higher in the moderate to severe NPDR group. VD differed significantly among groups (P = 0.003), whereas VTI was not significantly different (P = 0.22). Compared to the control group, VD was lower in moderate to severe NPDR and PDR groups. VD was also lower in the PDR group than that in the no DR group (P = 0.03). There was a significant correlation of VTI with SO2V (r = 0.32, P = 0.002) and OEF (r = -0.35, P = 0.001). Conclusions: Retinal vessel morphology, oxygenation, and tissue oxygen extraction were associated with each other in a cohort of subjects with and without DR. Translational Relevance: The findings of this study have the potential to improve clinical management of DR by providing better understanding of human disease pathophysiology and propelling future studies to identify multiple image-based biomarkers for improved disease diagnosis and monitoring.

The privacy of T cell memory to viruses.

T cell responses to viral infections can mediate either protective immunity or damaging immunopathology. Viral infections induce the proliferation of T cells specific for viral antigens and cause a loss in the number of T cells with other specificities. In immunologically naive hosts, viruses will induce T cell responses that, dependent on the MHC, recognize a distinct hierarchy of virus-encoded T cell epitopes. This hierarchy can change if the host has previously encountered another pathogen that elicited a memory pool ofT cells specific to a cross-reactive epitope. This heterologous immunity can deviate the normal immune response and result in either beneficial or harmful effects on the host. Each host has a unique T cell repertoire caused by the random DNA rearrangement that created it, so the specific T cells that create the epitope hierarchy differ between individuals. This "private specificity" seems of little significance in the T cell response of a naive host to infection, but it is of profound importance under conditions of heterologous immunity, where a small subset of a cross-reactive memory pool may expand and dominate a response. Examples are given of how the private specificities of immune responses under conditions of heterologous immunity influence the pathogenesis of murine and human viral infections.

Memory CD8+ T cells in heterologous antiviral immunity and immunopathology in the lung.

A potent role for memory CD8+ T cells in heterologous immunity was shown with a respiratory mucosal model of viral infection. Memory CD8+ T cells generated after lymphocytic choriomeningitis virus (LCMV) infection were functionally activated in vivo to produce interferon-gamma (IFN-gamma) during acute infection with vaccinia virus (VV). Some of these antigen-specific memory cells selectively expanded in number, which resulted in modulation of the original LCMV-specific T cell repertoire. In addition, there was an organ-selective compartmental redistribution of these LCMV-specific T cells during VV infection. The presence of these LCMV-specific memory T cells correlated with enhanced VV clearance, decreased mortality and marked changes in lung immunopathology. Thus, the participation of pre-existing memory T cells specific to unrelated agents can alter the dynamics of mucosal immunity and disease course in response to a pathogen.

Innate immunity to viruses: control of vaccinia virus infection by gamma delta T cells.

The existence of gammadelta T cells has been known for over 15 years, but their significance in innate immunity to virus infections has not been determined. We show here that gammadelta T cells are well suited to provide a rapid response to virus infection and demonstrate their role in innate resistance to vaccinia virus (VV) infection in both normal C57BL/6 and beta TCR knockout (KO) mice. VV-infected mice deficient in gammadelta T cells had significantly higher VV titers early postinfection (PI) and increased mortality when compared with control mice. There was a rapid and profound VV-induced increase in IFN-gamma-producing gammadelta T cells in the peritoneal cavity and spleen of VV-infected mice beginning as early as day 2 PI. This rapid response occurred in the absence of priming, as there was constitutively a significant frequency of VV-specific gammadelta T cells in the spleen in uninfected beta TCR KO mice, as demonstrated by limiting dilution assay. Also, like NK cells, another mediator of innate immunity to viruses, gammadelta T cells in uninfected beta TCR KO mice expressed constitutive cytolytic activity. This cytotoxicity was enhanced and included a broader range of targets after VV infection. VV-infected beta TCR KO mice cleared most of the virus by day 8 PI, the peak of the gammadelta T cell response, but thereafter the gammadelta T cell number declined and the virus recrudesced. Thus, gammadelta T cells can be mediators of innate immunity to viruses, having a significant impact on virus replication early in infection in the presence or absence of the adaptive immune response.

Independent regulation of lymphocytic choriomeningitis virus-specific T cell memory pools: relative stability of CD4 memory under conditions of CD8 memory T cell loss.

Infection of mice with a series of heterologous viruses causes a reduction of memory CD8(+) T cells specific to viruses from earlier infections, but the fate of the virus-specific memory CD4(+) T cell pool following multiple virus infections has been unknown. We have previously reported that the virus-specific CD4(+) Th precursor (Thp) frequency remains stable into long-term immunity following lymphocytic choriomeningitis virus (LCMV) infection. In this study, we questioned whether heterologous virus infections or injection with soluble protein CD4 Ags would impact this stable LCMV-specific CD4(+) Thp memory pool. Limiting dilution analyses for IL-2-producing cells and intracellular cytokine staining for IFN-gamma revealed that the LCMV-specific CD4(+) Thp frequency remains relatively stable following multiple heterologous virus infections or protein Ag immunizations, even under conditions that dramatically reduce the LCMV-specific CD8(+) CTL precursor frequency. These data indicate that the CD4(+) and CD8(+) memory T cell pools are regulated independently and that the loss in CD8(+) T cell memory following heterologous virus infections is not a consequence of a parallel loss in the memory CD4(+) T cell population.

Evolution of the CD8 T-cell repertoire during infections.

CD8 T cells exist in a dynamic network whose repertoire remains static in the absence of infection but changes in the presence of foreign antigens. Individuals each have unique T-cell repertoires that continually evolve in the presence of antigen and of cross-reactive heterologous antigens, and homeostatic forces drive deletions in T-cell memory pools to accommodate the entry of new memory cells into a finite immune system.

Attrition of T cell memory: selective loss of LCMV epitope-specific memory CD8 T cells following infections with heterologous viruses.

Using a variety of techniques, including limiting dilution assays (LDA), intracellular IFNgamma assays, and Db-IgG1 MHC dimer staining to measure viral peptide-specific T cell number and function, we show here that heterologous virus infections quantitatively delete and qualitatively alter the memory pool of T cells specific to a previously encountered virus. We also show that a prior history of a virus infection can alter the hierarchy of the immunodominant peptide response to a second virus and that virus infections selectively reactivate memory T cells with distinct specificities to earlier viruses. These results are consistent with a model for the immune system that accommodates memory T cell populations for multiple pathogens over the course of a lifetime.

Protective heterologous antiviral immunity and enhanced immunopathogenesis mediated by memory T cell populations.

A basic principle of immunology is that prior immunity results in complete protection against a homologous agent. In this study, we show that memory T cells specific to unrelated viruses may alter the host's primary immune response to a second virus. Studies with a panel of heterologous viruses, including lymphocytic choriomeningitis (LCMV), Pichinde (PV), vaccinia (VV), and murine cytomegalo (MCMV) viruses showed that prior immunity with one of these viruses in many cases enhanced clearance of a second unrelated virus early in infection. Such protective immunity was common, but it depended on the virus sequence and was not necessarily reciprocal. Cell transfer studies showed that both CD4 and CD8 T cell populations from LCMV-immune mice were required to transfer protective immunity to naive hosts challenged with PV or VV. In the case of LCMV-immune versus naive mice challenged with VV, there was an enhanced early recruitment of memory phenotype interferon (IFN) gamma-secreting CD4(+) and CD8(+) cells into the peritoneal cavity and increased IFN-gamma levels in this initial site of virus replication. Studies with IFN-gamma receptor knockout mice confirmed a role for IFN-gamma in mediating the protective effect by LCMV-immune T cell populations when mice were challenged with VV but not PV. In some virus sequences memory cell populations, although clearing the challenge virus more rapidly, elicited enhanced IFN-gamma-dependent immunopathogenesis in the form of acute fatty necrosis. These results indicate that how a host responds to an infectious agent is a function of its history of previous infections and their influence on the memory T cell pool.

Sympathoadrenal activation and muscarinic receptor stimulation induce acute release of tissue-type plasminogen activator but not von Willebrand factor across the human forearm.

We have previously shown that both mental stress and administration of the muscarinic receptor agonist methacholine induce an acute release of tissue-type plasminogen activator (t-PA) across the human forearm. There are data indicating that the regulated acute release of t-PA from the endothelium is closely interrelated with release of von Willebrand factor (vWF). The aim of the present study was to simultaneously determine basal and stimulated in vivo release rates of t-PA and vWF in an intact human muscle vascular bed. Eighteen healthy young males were studied at rest and during 10 min of mental stress (forced arithmetic). A subsample of ten subjects also received a step-wise i.a. infusion of methacholine (0.1-0.8-4.0 microg/min). Forearm blood flow was determined by venous occlusion plethysmography and interconverted to forearm plasma flow (FPF) using individual hematocrits. Net release/uptake rates of t-PA and vWF were calculated as the product of the arteriovenous concentration gradient and FPF. At rest there was a net release of both t-PA antigen and activity. In contrast, there was no significant local net release of vWF antigen across the forearm. Net release rates of t-PA roughly doubled in response to the stress test (0.4 to 0.8 and 0.2 to 0.5 ng x min(-1) x 100 ml(-1) for t-PA antigen and activity, respectively, p <0.05 for both). Local administration of methacholine induced a more than 10-fold increase in the net release rates of t-PA (0.6 to 9.6 and 0.3 to 6.6 ng x min(-1) x 100 ml(-1) at the highest dose step for antigen and activity respectively, p <0.01 for both). In contrast, neither mental stress nor local administration of methacholine induced a significant net release of vWF antigen across the forearm. The results demonstrate that the processes of acute release of t-PA and vWF are not necessarily linked in vivo in man.

Cytolytically active memory CTL present in lymphocytic choriomeningitis virus-immune mice after clearance of virus infection.

Generally, it has been assumed that memory T cells are dormant and inactive cells in the absence of their specific Ag. Recent work has challenged this assumption by showing that a portion of the CD8+ memory T cell pool is in cycle. In this study, we demonstrate that a significant number of blast-size memory CD8+ T cells in mice, long after lymphocytic choriomeningitis virus (LCMV) infection, mediate cytolysis against highly sensitive targets without any in vivo or in vitro restimulation and expansion with Ag. Peptide-coated RMA-S targets were sufficiently sensitive to detect low but significant cytolytic activity in bulk 51Cr release assays in nonstimulated LCMV-specific splenic memory CTL populations. Most of the directly cytotoxic activity was against the GP33 epitope, and this persisted throughout the lifetime of the mouse following infection. The cytotoxic activity was not inhibited by cyclosporin A, indicating that these cells were already in an active state and not dependent on further stimulation in vitro. It was formally shown that the cytotoxic activity was mediated by the CD8+ CTL by sorting for the blast-size CD8+ population and by blocking target cell lysis with anti-CD8 Ab. Thus, at any time after the original infection some portion of the memory CD8+ T cell pool is cycling, and it remains cytolytically active long after resolution of the original infection. These CTL may provide a rapidly acting defense mechanism against reinfection.

The analgesic efficacy of clodronate compared with placebo in patients with painful bone metastases from prostatic cancer.

Fifty-five patients with hormone refractory prostate cancer and painful bone metastases were randomised either to placebo or to clodronate 300 mg i.v. for 3 days, followed by oral clodronate 3200 mg for four weeks. Pain intensity was assessed using Visual Analogue Scales (VAS). Mean overall pain as well as mean pain during the best and worst periods were recorded. Forty-six patients were evaluable for efficacy. No significant differences were found between the two treatments. As regards mean worst pain a substantial numerical fall was registered for the treatment group, 21 mm, but the improvement was not significant compared to that of the placebo group. This was probably due to the limited number of patients (the study was prematurely ended due to problems recruiting patients). In conclusion, no significant differences were found between the treatment arm and the controls, in contrast to results from previous studies. Possible explanations are that the doses in this study were generally lower than in previous studies, the mean baseline pain was substantially lower and that the current study was placebocontrolled. Our data indicate that if clodronate is to be used for the alleviation of bone pain in prostate cancer, patients with high baseline should be selected and high intravenous doses should be given at start of the treatment.

Alpha beta and gamma delta T-cell networks and their roles in natural resistance to viral infections.

Both alpha beta and gamma delta T-cell populations and natural killer (NK) cells include cytotoxic, interferon (IFN)-gamma-producing lymphocytes that actively respond to viral infections. We show here that all three populations can provide "natural resistance" to viruses very early in infection and describe how the T-cell populations are modulated to provide this function. gamma delta T cells were shown to play a role in controlling vaccinia virus (VV) infections, as VV grew to much higher titers in gamma delta T-cell knockout mice than in normal mice 3-4 days post-infection. Our studies of the alpha beta T-cell responses to viruses revealed an interactive network of T cells that is modulated substantially during systemic infections. There is an induction phase associated with a massive virus-specific CD8 T-cell response, an apoptosis phase during which the T cells become sensitized to activation-induced cell death (AICD), a silencing phase, during which the T-cell number and activation state is reduced, and, finally, a memory phase associated with the very stable preservation of virus-specific memory cytotoxic T-lymphocyte precursors (pCTL). Infection of mice immune to one virus with a heterologous virus leads to a selective expansion of memory CTL cross-reacting between the two viruses, but, after homeostasis is again established, there is a quantitative reduction and qualitative alteration of memory to the first virus. Our results suggest that memory alpha beta T cells cross-reactive between heterologous viruses mediate both immunopathology and protective immunity at early stages of the second virus infection. Thus, memory alpha beta T cells can, like gamma delta T cells and NK cells, provide natural immunity to viral infections.

Reduction of otherwise remarkably stable virus-specific cytotoxic T lymphocyte memory by heterologous viral infections.

Experimental analyses of the acute cytotoxic T lymphocyte (CTL) response to viruses have focused on studying these infections in immunologically naive hosts. In the natural environment, however, viral CTL responses occur in hosts that are already immune to other infectious agents. To address which factors contribute to the maintenance and waning of immunological memory, the following study examined the frequencies of virus-specific CTL precursor cells (pCTL) not only using the usual experimental paradigm where mice undergo acute infections with a single virus, and in mice immune to a single virus, but also in immune mice after challenge with various heterologous viruses. As determined by limiting dilution assays, the pCTL frequency (p/f) per CD8+ T cell specific for lymphocytic choriomeningitis virus (LCMV), Pichinde virus (PV), or vaccinia virus (VV) increased during the acute infections, peaking at days 7-8 with frequencies as high as 1/27-1/74. Acute viral infections such as these elicit major expansions in the CD8+ T cell number, which has been reported to undergo apoptosis and decline after most of the viral antigen has been cleared. Although the decline in the total number of virus-specific pCTL after their peak in the acute infection was substantial, for all three viruses the virus-specific p/f per CD8+ T cell decreased only two- to fourfold and remained at these high levels with little fluctuation for well over a year. The ratios of the three immunodominant peptide-specific to total LCMV-specific clones remained unchanged between days 7 and 8 of acute infection and long-term memory, suggesting that the apoptotic events did not discriminate on the basis of T cell receptor specificity, but instead nonspecifically eliminated a large proportion of the activated T cells. However, when one to five heterologous viruses (LCMV, PV, VV, murine cytomegalovirus, and vesicular stomatitis virus) were sequentially introduced into this otherwise stable memory pool, the stability of the memory pool was disrupted. With each successive infection, after the immune system had returned to homeostasis, the memory p/f specific to viruses from earlier infections declined. Reductions in memory p/f were observed in all tested immunological compartments (spleen, peripheral blood, lymph nodes, and peritoneal cavity), and on average in the spleen revealed a 3 +/- 0.4-fold decrease in p/f after one additional viral infection and an 8.4 +/- 3-fold decrease after two additional viral infections. Thus, subsequent challenges with heterologous antigens, which themselves induce memory CTL, may contribute to the waning of CTL memory pool to earlier viruses as the immune system accommodates ever-increasing numbers of new memory cells within a limited lymphoid population. This demonstrates that virus infections do not occur in immunological isolation, and that CD8+ T cell responses are continually being modulated by other infectious agents.

Role of apoptosis in the regulation of virus-induced T cell responses, immune suppression, and memory.

Apoptosis is an important mechanism enabling the selection of the non-self-reactive T cell repertoire and for maintaining homeostasis in the immune system after it has expanded to combat infections. Highly activated, proliferating T cells become susceptible to apoptosis driven by a number of stimuli, and T cells activated during a viral infection become susceptible to "activation induced cell death" after repeated stimulation through the T cell receptor (TcR). This is a major mechanism for the immune deficiencies observed during many viral infections. During infections with a high antigen load this can lead to a selective deletion of virus-specific cytotoxic T lymphocytes (CTL) and to the establishment of persistent infection. More commonly, the CTL control the infection first, and high levels of apoptosis in the expanded lymphocyte population occur after antigen and growth factors become limiting. This cell death does not seem to depend on TcR specificity, as the residual population contains a remarkably stable population of memory CTL precursors that approximate the frequency per CD8 cell of that seen during the peak of the acute infection. Subsequent infections with heterologous viruses result in an expansion and then an apoptotic elimination of T cells, with the consequence being a reduction in precursor CTL specific for the first virus. Thus, apoptosis shapes the quality and quantity of T cell memory.

Protective CTL-dependent immunity and enhanced immunopathology in mice immunized by particle bombardment with DNA encoding an internal virion protein.

Anti-viral CTL were induced in vitro using a particle bombardment device or "gene-gun" to deliver plasmid DNA encoding the nucleoprotein of the lymphocytic choriomeningitis virus (LCMV). Using this plasmid we were able to study T cell-mediated immunity in the absence of a neutralizing Ab response. Upon a single DNA immunization, a nearly 2 log10 reduction in splenic viral titers was observed 3 days after LCMV infection. After two or three immunizations a greater than 3 log10 inhibition of viral titers in the spleen was observed, with most animals having no detectable virus. C57BL/6 mice immunized with DNA encoding the nucleoprotein gene were also challenged with LCMV intracranially. Upon intracranial challenge, vaccinated animals displayed either protection or enhanced immunopathology leading to accelerated kinetics of death. Using limiting dilution analysis it was possible to detect LCMV-specific CTL precursors in both the spleen and lymph nodes of vaccinated animals. C57BL/6 mice inoculated with DNA demonstrated an anamnestic CTL response detectable at day 4 after LCMV challenge. Thus DNA vaccines are capable of inducing an anti-viral T cell response that can inhibit viral replication and mediate either protective immunity or immunopathology. Vaccination with DNA may therefore provide a useful alternative to current viral or subunit vaccines once the efficacy of immunization with DNA is optimized.

Release of tissue-type plasminogen activator in response to muscarinic receptor stimulation in human forearm.

We have recently shown that mental stress increases local net release of tissue-type plasminogen activator (t-PA) across the forearm vascular bed. However, the mechanisms responsible for the t-PA release in man during stress are undefined. To study the effects of endothelial cell receptor stimulation and fluid shear stress we used the perfused forearm model to characterize the in vivo tissue plasminogen activator (t-PA) response in man to methacholine (Mch) and sodium nitroprusside (SNP), at doses calculated to cause similar degrees of vasodilation. The study was performed in 7 healthy young men (age 22-24 yrs) without hypertension, diabetes mellitus, or hypercholesterolemia. Each subject received double-blind step-wise i. a. infusions of Mch (0.1-0.8-4.0 micrograms/min) and SNP (0.5-2.5-10 micrograms/min) in randomized order. Each dose step was infused for 5 min. Forearm blood flow was assessed by plethysmography. Net release/uptake was expressed as the product of arterio-venous concentration gradient and forearm plasma flow. At pre-infusion baseline, there was a significant net release of t-PA antigen of approximately 0.9 ng x min-1 x 100 ml-1 and t-PA activity of 3.5 fmol x min-1 x 100 ml-1 across the forearm. I.a. infusion of Mch and SNP increased forearm blood flow from 1.9 to 14.9 and from 1.8 to 12.1 ml x min-1 x 100 ml-1, respectively (Mch vs SBP N.S.).(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo release of tissue-type plasminogen activator across the human forearm during mental stress.

We have previously shown that plasma levels of endothelium-derived tissue-type plasminogen activator (t-PA) increase during mental stress. The aim of the study was to investigate in vivo release in an intact human muscle vascular bed. Eleven healthy young males (22-36 yrs) were studied at rest and during 10 min of mental stress (forced arithmetic). Net release or uptake were assessed by arterio-venous (AV) concentration gradients across the forearm of t-PA antigen and t-PA activity, and plasminogen activator inhibitor antigen type 1 (PAI-1). Forearm blood flow was measured by venous occlusion plethysmography. At rest, there was a positive AV-difference of t-PA activity across the forearm indicating a net release of t-PA activity of approximately 3.7 fmol x min-1 x 100 ml-1 (Wilcoxon's signed rank test vs 0, p = 0.01). However, t-PA antigen showed a variable release pattern. On the average, there was a net release of 0.17 ng x min-1 x 100 ml-1 after 60 min of rest (Wilcoxon vs 0, p = 0.07). PAI-1 antigen showed net release at rest. In response to stress, forearm blood flow increased from 1.9 to 2.9 ml x min-1 x 100 ml-1 (ANOVA, p = 0.007), and net release of t-PA activity increased to 9.8 fmol x min-1 x 100 ml-1 (ANOVA, p = 0.01 compared with rest). Arterial and venous plasma t-PA levels also increased significantly during stress (ANOVA, p < 0.01). t-PA antigen showed a similar but less pronounced release pattern during stress.(ABSTRACT TRUNCATED AT 250 WORDS)

Specificity and editing by apoptosis of virus-induced cytotoxic T lymphocytes.

Recent studies have defined an immunological network by which the acute cytotoxic T-lymphocyte response to viral infection modulates or is modulated by the antigen load and by crossreactive memory T cells. Down regulation of the acute CTL response can be associated with either antigen-dependent or antigen-independent apoptosis, and the host enters a state of immune deficiency as these T cells become sensitized to apoptotic mechanisms.

Cross-reactivities in memory cytotoxic T lymphocyte recognition of heterologous viruses.

Analyses of the relationships between different viruses and viral proteins have focused on homologies between linear amino acid sequences, but cross-reactivities at the level of T cell recognition may not be dependent on a conserved linear sequence of several amino acids. The CTL response to Pichinde virus (PV) and vaccinia virus (VV) in C57BL/6 mice previously immunized with lymphocytic choriomeningitis virus (LCMV) included the reactivation of memory cytotoxic T lymphocyte (CTL) specific to LCMV. Limiting dilution assays (LDA) demonstrated that at least part of this reactivation of memory cells in LCMV-immune mice related to cross-reactivity at the clonal level, even though acute infections with these viruses in nonimmune mice elicited CTL responses that did not cross-react in conventional bulk CTL assays. Precursor CTL (pCTL) to LCMV were generated in splenic leukocytes from LCMV-immune mice acutely infected with PV or VV when stimulated in vitro with only the second virus but not with uninfected peritoneal exudate cells (PECs). Cytotoxicity mediated by LCMV-specific CTL clones activated by PV infection was greatly inhibited by anti-CD8 antibody, suggesting that these memory CTL clones recognizing LCMV-infected targets were of low affinity. LCMV-immune splenocytes stimulated in vitro with PV or VV demonstrated a low but significant precursor frequency (p/f) to the heterologous viruses, and splenocytes from PV- or VV-immune mice when stimulated in vitro against LCMV generated a low but significant p/f to LCMV. Short-term CTL clones cross-reactive between LCMV and PV were derived from splenic leukocytes from LCMV-immune mice acutely infected with PV. To distinguish whether the cross-reactivity was directed against a viral peptide or a virus-induced endogenous cellular neoantigen, we demonstrated that a pCTL frequency to PV about 1/4-1/7 that of the frequency to LCMV could be generated from LCMV-immune splenic leukocytes stimulated with the immunodominant LCMV NP peptide. A partially homologous PV peptide generated from the equivalent site to the LCMV NP peptide did not sensitize targets to lysis by either LCMV- or PV-specific CTLs, suggesting that the cross-reactivity in killing was not due to evolutionarily conserved equivalent sequences. Experiments also indicated that prior immunity to one virus could modulate future primary immune responses to a second virus. Elevated pCTL frequencies to PV were seen after acute PV infection of LCMV-immune mice, and elevated pCTL frequencies to LCMV were seen after acute LCMV infection of PV- and VV-immune mice.(ABSTRACT TRUNCATED AT 400 WORDS)